Highlighted Papers / Colloquium Papers

Highlighted Papers are articles selected by the Editorial Board to increase the visibility of what are deemed to be especially important papers. The full PDF of the Highlighted Papers can be viewed and downloaded from this page free of charge. Every Highlighted Paper is introduced by a short paragraph explaining the novelty and particular importance of the published work.

A further subset of Highlighted Papers is "Colloquium Papers". They describe the development of new areas of research or the impact of new and promising experimental or theoretical methods in the fields that are within the spectrum of topics covered by the journal. While not as extensive and complete as reviews in the usual sense, they are intended to suitably introduce new research directions and techniques in their early stage of development to a wider audience.

January 2012

EPJE - How to build doughnuts with Lego blocks

Complex polymer rings with breathtaking nanoscale architecture revealed
Scientists have uncovered how nature minimises energy costs in rings of liquids with an internal nanostructure made of two chemically discordant polymers joined with strong bonds, or di-blocks, deposited on a silicon surface, in an article published in EPJE.
Josh McGraw and his colleagues from McMaster University, Canada, and the University of Reading, UK, first created rings of di-block polymers that they liken to building doughnuts from Lego blocks due to the nature of the material used. This material has an internal structure discretised like Lego blocks, resulting in rings approximating the seamless shape of a doughnut (see photo of previously unseen nanoscale assemblies which illustrates this press release).
McGraw and his colleagues measured the dynamics of interacting edges in ring structures that display asymmetric steps, i.e., different spacing inside and outside the ring, when initially created. They found that the interaction shaping the ring over time is the repulsion between edges. While the molecular details remain elusive, the source of this repulsion is intuitive: an edge is a defect which perturbs the surface profile with an associated cost to the surface energy.
The edge repulsion prevents two neighbouring edges from getting too near each other. As two isolated edges approach, the perturbation deviates further, thereby deforming the equilibrium edge structure and increasing the free energy. For rings solely subject to the repulsive edge interaction, the authors found that the equilibrium shape of their edges had to be symmetric.
These edges could be considered defects in a material with an otherwise perfect order at the nanoscale. Thus, research based on the elucidation of defect interactions could help scientists trying to eliminate such defects by understanding how these materials self-assemble. Such systems could also provide an ideal basis for creating patterns on the nanoscale, data storage, and nanoelectronics.

Dynamics of interacting edge defects in copolymer lamellae. J.D. McGraw, I.D.W. Rowe, M.W. Matsen, and K. Dalnoki-Veress, Eur. Phys. J. E (2011) 34: 131, DOI 10.1140/epje/i2011-11131-7

December 2011

EPJE - The art of stabilising entangled spaghetti-like materials

Controlling forces between oppositely charged polymers opens a new route towards creating vectors for gene therapy
Gene therapy can only be effective if delivered by a stable complex molecule. Now, scientists have determined the conditions that would stabilise complex molecular structures that are subject to inherent attractions and repulsions triggered by electric charges at the surfaces of the molecules, in a study published in EPJE, by Valentina Mengarelli and her colleagues from the Solid State Physics Laboratory at the Paris-Sud University in Orsay, France, in collaboration with Paris 7 and Evry Universities scientists.
The authors studied soluble complexes made of negatively charged DNA or another negatively charged polymer – polystyrene-sulfonate (PSSNa) – and a so-called condensation agent, which is a negatively charged polymer, known as linear polyethyleneimine (PEI). PEI participates in the condensation process by tying onto a molecule such as DNA, like tangled hair, to form an overall positively charged DNA/polymer complex structure.
Previous research focused mainly on non-soluble complexes, while the few attempts at focusing on soluble complexes dealt either with smaller polymers or those with a weaker electric charge, which may therefore be easier to stabilise.
The French team thus confirmed experimentally that the complexation process does not depend on the rigidity of the original molecule, be it DNA or PSSNa, but on the positive/negative electric charge ratio and on the polymer concentrations. It is the interactions between electrically charged parts within the complex that govern its properties. When the condensation agent is in excess, the positively charged complex is then attracted to negatively charged biological cell membranes. This could be used to deliver the DNA into a targeted cell nucleus as part of gene therapy treatment.
Future work will focus on using long DNA molecules and novel polymers to form complexes of controlled size and electric charge for gene therapy.

Charge inversion, condensation and decondensation of DNA and Polystyrene sulfonate by polyethylenimine. V. Mengarelli et al., Eur. Phys. J. E (2011) 34: 127, DOI 10.1140/epje/i2011-11127-3

November 2011

EPJE - How biological capsules respond under stress

Innovative high-precision measuring tool to assess the bending elasticity of liposomes
Cosmetics and pharmaceutical drug delivery systems could be improved thanks to a new method developed to precisely measure the capability of capsule-like biological membranes to change shape under external stress. This work is outlined in a study published in EPJE by Philippe Méléard and Tanja Pott from the Rennes-based Institute of Chemical Sciences at the European University of Brittany and their colleagues from the Center for Biomembrane Physics at the University of Southern Denmark in Odense.
The authors found that, by using a statistical method, they could evaluate the bending elasticity of biological membrane models, a key factor in understanding their physical properties. They relied on a series of video-microscopy images of giant liposomes, which are artificial spherical vesicles of more than 10 micrometers in diameter made of a bi-layer of fatty substance called lipids. They studied the membrane deformations triggered by thermal agitation of molecules in the liquid surrounding them, over time.
Previous approaches used the average of deformation amplitudes observed in these images, which meant a loss of accuracy of up to 20%. Instead, in this study, the authors focused on evaluating the statistical distribution of the membrane deformation, which yielded unprecedented precision. This method relies on the so-called Maxwell-Boltzman statistical distribution, named after James Clerck Maxwell and Ludwig Boltzmann, who studied the kinetic theory of gas using this approach.
The method presented in this paper could be of interest to industry scientists in devising both cosmetic and pharmaceutical applications. For example, industry often needs to encapsulate products such as cytotoxic cancer drugs or antimicrobial peptides in biological membranes prior to delivering them into patients’ bodies. Ultimately, it could help industry scientists determine what type of biological membrane is best suited for their specific purpose.

Advantages of statistical analysis of giant vesicle flickering for bending elasticity measurements. P. Méléard et al., Eur. Phys. J. E (2011) 34: 116, DOI 10.1140/epje/i2011-11116-6

October 2011

EPJ E - How do protein binding sites stay dry in water?

In a report that has just been published in EPJE, researchers from the National University of the South in Bahía Blanca, Argentina studied the condition for model cavity and tunnel structures resembling the binding sites of proteins to stay dry without losing their ability to react, a prerequisite for proteins to establish stable interactions with other proteins in water.
E.P. Schulz and colleagues used models of nanometric-scale hydrophobic cavities and tunnels to understand the influence of geometry on the ability of those structures to stay dry in solution.
The authors studied the filling tendency of cavities and tunnels carved in a system referred to as an alkane-like monolayer, chosen for its hydrophobic properties, to ensure that no factors other than geometrical constraints determine their ability to stay dry.
They determined that the minimum size of hydrophobic cavities and tunnels that can be filled with water is on the order of a nanometer. Below that scale, these structures stay dry because they provide a geometric shield; if a water molecule were to penetrate the cavity it would pay the excessive energy cost of giving up its hydrogen bonds. By comparison, water fills carbon nanotubes that are twice as small (but slightly less hydrophobic) than the alkane monolayer, making them less prone to stay dry.
The authors also showed that the filling of nanometric cavities and tunnels with water is a dynamic process that goes from dry to wet over time. They believe that water molecules inside the cavities or tunnels are arranged in a network of strong cooperative hydrogen bonds. Their disruption by means of thermal fluctuations results in the temporary drying of the holes until new bonds are re-established.
One of the many potential applications is in biophysics, to study water-exclusion sites of proteins, and understand the physical phenomenon linked to the geometry of those sites, underpinning the widespread biological process of protein-protein associations.

Behavior of water in contact with model hydrophobic cavities and tunnels and carbon nanotubes. E.P. Schulz et al., Eur. Phys. J. E (2011) 34: 114, DOI 10.1140/epje/i2011-11114-8

October 2011

EPJ E - What makes tires grip the road on a rainy day?

Scientists examine the flow of liquid at the contact between randomly rough surfaces
A team of scientists from Italy and Germany has recently developed a model to predict the friction occurring when a rough surface in wet conditions (such as a road on a rainy day) is in sliding contact with a rubber material (such as a car tire tread block) in an article that has just been published in EPJE.
In their study, B.N.J. Persson from the Jülich Research Center in Germany and M. Scaraggi from the Polytechnic of Bari in Italy examined the flow of liquid at the contact between randomly rough surfaces. The contact interface looks like a labyrinth with vertically narrow void channels intersecting randomly. This causes channels to be either filled with water or not when in wet conditions.
For the first time, the authors applied a statistical analytical method to determine the average fluid flow at the interface of rough surfaces. Understanding this flow is important because it is inherently linked to the phenomenon of friction at the contact between the two surfaces.
Previous attempts to understand friction in such conditions used numerical approaches that required large computing power. They were based on calculating real roughness contacts by singling out each individual portion of the overall rough surface under study. Often, heavy approximations in the description of the simulated surface were applied to decrease the computational time. The model presented in this paper provides theoretical predictions of friction as a function of the surface sliding velocity. It confirms previous experimental friction measurements made with a smooth steel ball sliding on a rough rubbery surface patterned with parallel grooves. The authors’ model confirmed the experimental observation of a changing friction level related to a change in the angle between the direction of movement of the ball and the parallel to the grooves.
Potential applications would require that such a model be used to help create surfaces, such as microstructured tires, which do not lower their grip when it rains.

Lubricated sliding dynamics: flow factors and Stribeck curve. B.N.J. Persson and M. Scaraggi, Eur. Phys. J. E (2011) 34: 113, DOI 10.1140/epje/i2011-11113-9

October 2011

EPJ E - Unlocking jams in fluid materials

A new theoretical model which helps to understand how to best avoid jamming of soft matter
In a study recently published in the European Physical Journal E (EPJE), a German scientist constructed a theoretical model to understand how to best avoid jamming of soft matter that can be applied in food and cosmetics production.
Thomas Voigtmann, a researcher at the Institute for Material Physics in Space in Cologne, Germany, evaluated the internal friction force, or yield stress, to be overcome before a solid material made of a metallic melt with a glass structure can flow and thus prevent jamming.
These materials have an apparent viscosity that drops if they are forced to flow quickly – a property called shear thinning. They are similar to solid paint that is highly viscous, almost solid, in a bucket and can easily become liquid when applied with a brush. The force applied to the paint by a brush stroke is sufficient for shear thinning to occur.
The properties of these metallic melts are not well understood. Until now, these materials have been studied using models for three classes of materials: soft matter (like toothpaste), metallic liquids, or granular materials (like sand).
However, none of these models accurately describes these materials. Instead, Voigtmann devised two models that take into account the common properties between the three material classes; here the goal was to determine whether their yield stress is either continuous (it gets smaller with the flow rate) or discontinuous (remains at a constant value regardless of the flow rate) at a decreasing flow rate. He used available data to test the models; however, further data on lower flow rates than currently available would be required in order to be conclusive.
Further theoretical research will help us to understand how to process large amounts of soft matter for the food industry such as mayonnaise (an emulsion), jelly (a colloidal dispersion), or granular materials such as grains or pharmaceutical pills while avoiding blockages as they flow through processing pipes.
This paper is part of a topical issue of EPJE dedicated to the “Physics of Glasses” edited by Michael Falk, Takeshi Egami and Srikanth Sastry published as European Physical Journal E (EPJE) Volume 34, number 9 (September).

Yield Stresses and Flow Curves in Metallic Glass Formers and Granular Systems. Th. Voigtmann, Eur. Phys. J. E (2011) 34: 106, DOI 10.1140/epje/i2011-11106-8

September 2011

EPJ E - New complex offers potentially safer alternative for gene therapy delivery

Spontaneous ordering of DNA fragments in a special matrix holds the key to creating non-toxic gene therapy delivery vectors, according to a study recently published in the European Physical Journal E. Scientists from the CNRS Paul Pascal Research Centre, an institute of the University of Bordeaux, France, and colleagues from the Institute of Physics at the University of Sao Paolo, have created a complex system designed to hold DNA fragments in solution between the hydrophilic layers of a matrix of fatty substances (also known as lipids) combined with a surfactant (used to soften the layers’ rigidity). One possible application that has yet to be tested is gene therapy.
Although gene therapy was initially delivered using viral vectors, recent attempts at devising alternative vectors have exploited positively charged lipids to form complex structures holding DNA fragments with electrostatic forces. However the positively charged ions, known as cations, used in this type of vector have proven toxic for human cells.
Until now, only positively charged fatty substance were thought capable of holding DNA in a complex vector. The authors of this study have proved otherwise by creating an electrically neutral matrix, structured like a multi-layered cake, which holds the DNA fragments at a high concentration in solution between the layers.
The authors found that DNA fragments within the complex self-organise over time. These fragments spontaneously align parallel to one another and form rectangular and hexagonal structures across the layers. The change of atomic-level interactions within the layers and the appearance of interactions at the interface between the layers and the DNA molecules may explain the emergence of ordered structures at high DNA concentrations.
The next step of this research involves elucidating the precise physical forces that hold the complex together. Applications of such technology go beyond gene therapy vector design, as the same principle can be applied for the delivery of other particles such as chemical drugs.

Supramolecular polymorphism of DNA in non-cationic L_α lipid phases. E.R. Teixeira da Silva et al., Eur. Phys. J. E (2011) 34: 83

July 2011

EPJ E - Embryo development obeys the laws of hydrodynamics

The law of hydrodynamics can contribute to our understanding of how a cluster of embryonic cells can transform into an animal within the first 36 hours of development, according to research recently published in European Physical Journal E. Vincent Fleury, a researcher at the Paris Diderot University, studied the early stage of development when embryonic cells first form a flat sheet of cells before folding into a U-shape, resembling a folded pancake. He demonstrated that the formation of a chicken’s head is a consequence of the collision between both sides of the embryo flowing at constant speed towards each other.
This study captured for the first time on film highly accurate observations of how a chicken embryo evolves during its first two days of development, using time-lapse microscopy. Prior attempts relied on complex imaging techniques that were costly and not as accurate as direct filming. In this study, the embryo was first taken out of its shell, its yolk removed (as it is not needed in the first 48 hours) and it was kept under appropriate temperature conditions.  
Previous developmental studies focused on studying each cell individually. In this study, the embryo was considered in its entirety, like a type of plasticine material able to flow like Dali’s melting clocks. The study involved measuring the speed of all points of the embryo and its viscoelasticity in vivo. Combining this data with the biological parameters of the embryo (cells’ viscosity, thickness and overall size), the author created a model of the growing embryo’s movement.
He discovered that the mathematical formula describing magnetic fields could also be used to model fields of vectors representing the hydrodynamic flow of embryonic cells. When the two sides collided, the embryonic cells were subject to forces that can be described as those of two magnets oriented head on, which resulted in the formation of the head.
These findings demonstrate that the head formation does not merely result from a series of discrete events activated by genetic switches. It also shows that chemical gradients are not the prevailing force responsible for movement of cells in early embryo formation, as had been previously thought.
These studies shed new light to on vertebrate development, and could ultimately provide some clues for scientists involved in regenerative medicines.
Similar work on limb development is due to be published in the August issue of the European Physical Journal Applied Physics.

A change in boundary conditions induces a discontinuity of tissue flow in chicken embryos and the formation of the cephalic fold. V. Fleury, Eur. Phys. J. E (2011) 34: 73

Additional videos are available at: http://www.msc.univ-paris-diderot.fr/~vfleury/embryoportal0.html

July 2011
Dynamical behavior of molecular motor assemblies in the rigid and crossbridge models
In cells, motor proteins use chemical energy to generate motion and forces. Motors often interact and form clusters because they are connected to a single rigid backbone. In a muscle the backbone is made by association of the motor tails. The backbone motion results from the action of all the motors, and feeds back on each motor. Previous works suggest that motor assemblies are endowed with complex dynamical properties, including dynamic instabilities and spontaneous oscillations, which may play a role in the mechanisms of heartbeat, flagellar beating, or hearing. In this paper, we study two models of motor assemblies: the rigid two-state model and the classical crossbridge model widely used in muscle physiology.
Both models predict spontaneous oscillations. In the rigid two-state model, they can have a "rectangular" shape or a characteristic "cusp-like"' shape that resembles cardiac sarcomere and "stick-slip" oscillations. The oscillations in the vicinity of the Hopf bifurcation threshold can be much faster than the chemical cycle. This property, not found in the crossbridge model where protein friction slows down the motion, could be important for the description of high frequency oscillations, such as insect wingbeat. Experiments based on the response of a motor assembly to a step displacement are also well described by both theories, which predict non-linear force displacement relations, delayed rise in tension and "sarcomere give"'. This suggests that these effects are not directly dependent on molecular details. We also relate the collective properties of the motors to their microscopic properties accessible in single molecule experiments: we show that a three state state crossbridge model predicts the existence of instabilities even in the case of an apparent load decelerated detachment rate.

Figure caption: Examples of spontaneous oscillations of motor assemblies in the crossbridge model (red) and the rigid model (blue).

To read the full paper “Dynamical behavior of molecular motor assemblies in the rigid and crossbridge models” by Thomas Guérin, Jacques Prost, and Jean-François Joanny, Eur. Phys. J. E (2011) 34: 60, click here.

July 2011
Secondary structure formation of homopolymeric single-stranded nucleic acids including force and loop entropy: Implications for DNA hybridization
Loops are essential secondary structure elements in folded DNA and RNA molecules and proliferate close to the melting transition. Using a theory for nucleic acid secondary structures that accounts for the logarithmic entropy -c ln m for a loop of length m, we study homopolymeric single-stranded nucleic acid chains under external force and varying temperature. In the thermodynamic limit of a long strand, the chain displays a phase transition between a low temperature / low force compact (folded) structure and a high temperature / high force molten (unfolded) structure. The influence of c on phase diagrams, critical exponents, melting, and force extension curves is derived analytically. For vanishing pulling force, only for the limited range of loop exponents 2 < c < 2.479 a melting transition is possible. A force induced melting transition with singular behavior is possible for all loop exponents c < 2.479 and can be observed experimentally by single molecule force spectroscopy. These findings have implications for the hybridization or denaturation of double stranded nucleic acids. The Poland-Scheraga model for nucleic acid duplex melting does not allow base pairing between nucleotides on the same strand in denatured regions of the double strand. If the sequence allows these intra-strand base pairs, we show that for a realistic loop exponent c ~ 2.1 pronounced secondary structures appear inside the single strands. This leads to a lower melting temperature of the duplex than predicted by the Poland-Scheraga model.

Figure caption: Phase diagrams displaying a folded (native) and an unfolded (denatured) RNA phase in the w-c plane with and without applied force; w is the Boltzmann factor of base pairing and c is the loop exponent. The dotted line depicts the classical Poland-Scheraga result for the melting of a double-stranded nucleic acid chain.

To read the full paper “Secondary structure formation of homopolymeric single-stranded nucleic acids including force and loop entropy: Implications for DNA hybridization” by T.R. Einert, H. Orland, and R.R. Netz, Eur. Phys. J. E (2011) 34: 55, click here.

March 2011
Crystal nucleation at the surface of polymer droplets
A nucleation site initiates the birth of a crystal. In most cases, take for example the dust particle in a snowflake, nucleation starts from a heterogenous defect. Homogenous nucleation is more elusive because of the prevalence of defects in any bulk sample. Crystallisation in tiny droplets alleviates this difficulty in a manner that is conceptually simple: subdivide the system into more domains than the number of defects. If the domains greatly outnumber the defects then only the homogenous mechanism can induce nucleation in a defect free compartment. Such an approach has been used here to investigate nucleation in polyethylene (PE) droplets. At high temperatures, a thin PE film dewets from an unfavourable surface forming tiny droplets, much like water beading up on a waxy leaf (Fig. (b)). The resulting sample geometry is ideal: thousands of droplets ranging in size can be monitored simultaneously with optical microscopy, with a nucleation event easily distinguishable by the rapid growth of the crystal (Fig. (c)). Each droplet becomes an isolated independent nucleation experiment. By investigating thousands of droplets supercooled well below the melting temperature, studies of homogenous nucleation become straightforward. Relating the probability of homogenous nucleation to the size of the droplet, the authors show that nucleation is surface activated. Stated most simply, a droplet with twice the surface area is twice as l ikely to nucleate, indicating that the perturbation induced by the interface reduces the intrinsic activation barrier to crystal nucleation.

Figure caption: Caption: a) A Si substrate with a polystyrene (PS) layer forms an unfavourable surface for a thin PE film. b) Upon heating, the unstable film dewets to form droplets. c) Optical microscopy image (500 μm wide). Amorphous droplets appear dark, while crystalline droplets become bright.

To read the full paper “Surface nucleation in the crystallisation of polyethylene droplets” by J.L. Carvalho and K. Dalnoki-Veress, Eur. Phys. J. E 34, 6 (2011), click here.

February 2011 / Colloquium Paper
Recent trends in the tuning of polymersomes' membrane properties
Polymersomes, fascinating vesicular structures self-assembled from amphiphilic block copolymers, gave rise to an increased research activity over the past decade and impacted a large scientific community. Chemists, physical chemists, biophysicists but also an increasing number of biologists, radiologists or pharmacologists are currently working on polymersomes in various contexts such as drug delivery, medical imaging, micro-reactors or to mimic biophysical phenomena of membranes such as adhesion, fusion, fission, motility, photosynthesis ...In all these researches, the guiding thread is modulation of membranes' properties both from physical, structural and functional points of view.

In this colloquium paper J-F. Le Meins, O. Sandre and S. Lecommandoux from the "Laboratoire de Chimie des Polymères Organiques" (CNRS, University of Bordeaux) present the molecular (coil-coil, rod-coil, dendrimeric block copolymer...) and formulation based methodologies (protein insertion, gelification of internal cavity...) aimed at tuning their mechanical and permeability properties. Very recent and promising blend approaches to create hybrid (inorganic/organic or organic/organic) membranes are also highlighted.

J.-F. Le Meins, O. Sandre, and S. Lecommandoux, Recent trends in the tuning of polymersomes' membrane properties, Eur. Phys. J. E (2011) 34, 14]

February 2011 / Colloquium Paper
RAFT "grafting-through" approach to surface-anchored polymers: Electrodeposition of an electroactive methacrylate monomer
The formation of polymer brushes is of high interest for smart materials and coatings. By "grafting-through" a methacrylate-functionalized conducting polymer film made of electropolymerized carbazole dendrons and subsequent polymerization with reversible addition-fragmentation chain transfer (RAFT), new routes to novel coatings can be realized. The conducting polymer film was electrodeposited over a conducting surface, i.e. gold or indium tin oxide (ITO)) using cyclic voltammetry (CV). This film was then used as the surface for a RAFT polymerization process of methyl methacrylate (MMA), styrene (S), and tert-butyl acrylate (TBA), resulting in grafted polymer chains. These types of films are useful for distinct polymer multilayers made of electro-optically active conducting polymers and insulating vinylic and functional polymers that are chemically bound. Possible uses of such films are for display devices, sensors, anti-corrosion coatings, controlled wetting surfaces, and anti-static materials.

[C.D. Grande et al. RAFT "grafting-through" approach to surface-anchored polymers: Electrodeposition of an electroactive methacrylate monomer, Eur. Phys. J. E (2011) 34, 15]

December 2010
EPJ E - How the fruit fly got its spots
Biology provides the physicist with a stunning variety of patterns to explore, and several fundamental ideas in the physics of pattern formation, such as Turing instabilities and the clock-and-wavefront mechanism, are rooted in studies of biological systems. It remains unclear, however, whether these classic concepts can explain the emergence of patterns in most biological systems, or whether new and different mechanisms remain to be discovered.
In a recent paper in EPJE, Matthew Pennington and David Lubensky of the University of Michigan at Ann Arbor examined a spatially discrete, three variable reaction-diffusion model inspired by the interactions that create a periodic pattern of gene expression in the Drosophila eye imaginal disc. This model is capable of creating a regular pattern behind a moving front, as observed in eye discs, through a novel “switch and template” mechanism. In order to better understand this mechanism, the authors performed a detailed study of the model’s behavior in one dimension, using a combination of analytic methods and numerical searches of parameter space. Using this approach, the authors find that patterns are created robustly, provided that there is an appropriate separation of time scales and that self-activation is sufficiently strong. Moreover, the paper presents explicit expressions in this limit for the front speed and the pattern wavelength. Moving fronts in pattern-forming systems near an initial linear instability generically select a unique pattern, but the P&L model operates in a strongly non-linear regime where the final pattern depends on the initial conditions as well as on parameter values. This study highlights the important role that cellularisation and cell-autonomous feedback can play in biological pattern formation.
[M.W. Pennington and D.K. Lubensky. Switch and template pattern formation in a discrete reaction-diffusion system inspired by the Drosophila eye. Eur. Phys. J. E, 33, 2010, 129-148].

September 2010
Drag forces in fluctuating classical field
Identical objects in thermally fluctuating fields experience a fluctuation induced force between them, examples include the famous critical Casimir force (generated by thermal rather than quantum fluctuations) [1] and forces induced between proteins in lipid membranes via their coupling to membrane height or composition [2] degrees of freedom. Gaussian fields, linearly coupled to the position of a moving inclusion the field can also induce a drag force [3]. The underlying physics is similar to that of a polaron - for a stationary inclusion, the polarization of the field is spherically symmetric, however when it moves the polarization field is deformed, see Figure 1, and this deformation yields a drag. The drag force depends on the statics and dynamics of the field and the inclusion's interaction with the field. At low velocities v the drag force is generically linear in v, but for systems with long range correlations, such as fields at critical points (for instance the continuous demixing transition for lipid membranes), the drag force can behave nonanalytically as v^Ф, where Ф<1. As the velocity is increased, the drag force increases to a maximum and then decays to zero as 1/v. This is because at high velocity the polarization cloud does not have sufficient time to develop and the drag is thus reduced. These effects could be measured experimentally, for example on membrane proteins dragged in membranes or on colloids dragged through binary liquid mixtures, using optical tweezers.
[1] M.E. Fisher and P.-G. de Gennes, C. R. Acad. Sci. Paris B 287, 207 (1978)
[2] M. Goulian, R. Bruinsma, and P. Pincus, Europhys. Lett. 22, 145 (1993).
[3] V. Démery and D.S. Dean, to appear Eur. Phys. J. E 32 (2010)
[Vincent Démery and David S. Dean, Eur. Phys. J. E (2010)]

June 2010
Optohydrodynamics of soft fluid interfaces
Are we able to control and actuate dynamically the shape of a fluid interface at a microscopic scale? Among the various methods (dielectrophoresis, electrowetting, ...), the recent interest in optofluidics, eg methods based on the combination of optics and fluidics, promoted innovative approaches using the optical radiation pressure of laser beams to manipulate liquid interfaces. Since flows are optically driven, we call this emerging field optohydrodynamics. Beyond exciting academic insights, optohydrodynamics is involved in many interesting applications ranging from interface rheology to adaptive optics or surface relief micropatterning. We present here an example of optohydrodynamic actuation based on experimental and predictive numerical results (using the Boundary Element Method) which show that the bending of a fluid-fluid interface strongly depends on the refractive index contrast between the two fluids. The characteristic time required to reach equilibrium increases when decreasing this contrast while equilibrium shapes of the deformation switch from a needle-like to a nearly-cylindrical finger. The physical feature at the origin of these behaviors lies on the nonlinear dependence of the optical radiation pressure on the local incidence angle. The viscosity ratio between the two fluids also affects the dynamics of large scale deformations. This investigation illustrates one of the simplest manifestations of optohydrodynamics and provides a frame to anticipate further developments of contactless interface micromanipulation by lasers.
[H. Chraibi et al., Eur. Phys. J. E (2010)]

March 2010
Liquid crystals straighten up
Discotic columnar liquid crystals have a remarkable capability to transport charge in just one direction, along the columns formed by stacks of their flat, aromatic, disk like molecules. In the mesophases formed by these materials, these columns are arranged in a two-dimensional crystal lattice. These materials are potentially useful for organic solar cells, but to achieve good performance from such devices (which strongly depends on the quality of charge and exciton diffusion in the materials used) one needs to be able to prepare uniform thin films on conductive substrates with the axis of good transport along the columns vertical (this is called homeotropic orientation). The difficulties of achieving this are stressed in the paper by Grelet et al., which describes structural investigations by grazing incidence X-ray diffraction on thin films of columnar liquid crystals. This work shows that a strong planar orientation (with columns parallel to the surface) is found for a very wide variety of discotic compounds, film preparation processes, film thicknesses, and types of solid substrate. This degenerate planar alignment corresponds to the worst orientation for carrying charges or excitons in organic devices, and can by explained by anchoring energy considerations. Nevertheless, the authors have discovered a specific thermal process that provides a convenient way to achieve homeotropic anchoring of hexagonal columnar liquid crystal films, which is the suitable alignment for photovoltaic devices.
[E. Grelet et al., Eur. Phys. J. E (2010)]

November 2009
Adhesion and membrane tension of cells
To understand many biological processes, and the interaction between cells and materials, we need to understand the way cells stick to each other and to substrates. A new technique to characterise this adhesion combines the best attributes of two previously developed methodologies, the micropipette aspiration techniques and the use of atomic force microscopy. As in the micropipette technique, slight suction is applied to a pipette that is ~ 10 μm in diameter in order to capture a single cell. A pipette is bent into a long thin L-shaped cantilever so that, as in an AFM, the force felt by the cell as it interacts with the substrate can be measured by the micropipette deflection (MD); this method has the advantage over the classical micropipette technique that the force is known through-out a binding-unbinding experiment. We have tested the MD methodology with measurements on a model cell, a liposome, and living cells. Interestingly, measurements reveal that the relaxation of a living cell as it is squeezed between the pipette and a substrate is logarithmic. Such relaxation has been seen in other strongly interacting complex systems such as granular materials, spin-glasses and proteins. The MD methodology can be applied to a wide variety of systems: cell-substrate or cell-cell, and any other systems that can be manipulated with the micropipette like colloidal beads, fibers, microtubules, and aggregates. Furthermore, by scanning a sample it is possible to carry out both friction measurements and force imaging.
[M.-J. Colbert et al., Eur. Phys. J. E 30, 117-121 (2009)]

March 2009
Thermomechanical Lehmann effect in cholesteric liquid crystals
In 1900, Otto Lehmann observed the continuous rotation of cholesteric droplets when heated from below. This thermomechanical phenomenon was explained 68 years later by Leslie from symmetry arguments. According to the theory, the director experiences a torque proportional to the temperature gradient. The proportionality constant is called the Lehmann coefficient. So far, this coefficient has only been measured close to the compensation point of very special mixtures (see for instance N. Eber and I. Janossy, Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982), and P. Oswald and A. Dequidt Europhysics Lett. 83, 16005 (2008)). It was thus important to extend such measurements to more usual cholesterics. In this context, we used a standard nematic liquid crystal (eutectic mixture of cyanobiphenyls 8CB and 8OCB) doped with a small amount of the chiral molecule R811. We observed the Lehmann rotation of cholesteric droplets subjected to a temperature gradient. This experiment - not reproduced to our knowledge since Lehmann's original work - showed that the angular velocity of the droplets strongly depends on their size and on the concentration of chiral molecules. To estimate the Lehmann coefficient, three different methods were used. The first one consisted of measuring the droplet angular velocity as of function of the droplet size. The second one consisted of applying an electric field to stop the droplet rotation. The last one consisted of observing below which critical size the drops stop rotating because of a textural change. The three methods led to consistent values of the Lehmann coefficient at the clearing temperature. In addition, it was found that the coefficient is proportional to the concentration of chiral molecules.
[P. Oswald, Eur. Phys. J. E 28, 377-383 (2009)]

February 2009
Oscillatory dynamics induced in polyelectrolyte gels by a non-oscillatory reaction: A model.
Can a gel spontaneously oscillate in size? We have shown theoretically that a polyelectrolyte gel immersed in a reacting medium, kept far from equilibrium by a constant feed of fresh reactants, can change shape with a regular periodicity. What is needed is a hydrogel which can swell or shrink as a function of the chemical composition of their solvent, such as a polyelectrolyte in response to pH changes. Combining such a gel with an autocatalytic chemical reaction can lead, through the coupling and mutual feedback of concentration, diffusion and swelling/shrinking, to an oscillatory instability, even when the surrounding medium is at stationary state. Our model accounts for the main transport, reaction, and swelling processes involved. The prototypical Bromate-Sulfite reaction exhibits an autocatalysis with H^+ . Whereas this non oscillatory reaction could only lead to stationary concentration distributions in an inert gel, we predict that, in a narrow domain of size, it can induce periodic mechanical and chemical pulsation in a pH sensitive polyelectrolyte gel: although the composition of the bath is stationary, the gel size and the composition in the core oscillate. These results, which are supported by preliminary experiments, pave the way for objects capable of autonomous motion driven by the chemical environment.
[J. Boissonade, Eur. Phys. J. E 28, 337-346 (2009)]

October 2008
Local friction at a sliding interface between an elastomer and a rigid spherical probe
Friction is one of the most longstanding problems in physics. One of the major origins of the complexity of this problem comes from the roughness of the contacting surfaces. When macroscopic bodies are pressed together, contact only occurs at localized spots between surface asperities. Friction thus involves the shearing of a myriad of micro-contacts which are distributed over length scales ranging from micrometers down to nanometers. Although widely debated, the manner in which these micro-contacts locally dissipate energy remains obscure. As a prerequisite, one should know how frictional stresses are distributed within the highly heterogeneous stress and strain field of macroscopic contact interfaces. Unfortunately, most experiments only rely on measurements of friction force and of its dependence on load and velocity which are averaged quantities of local frictional properties. We recently proposed a method to measure local friction of rubbers by means of a contact imaging approach. Silicon rubber substrates marked beneath their surface by a coloured pattern were prepared in order to measure the displacement field induced by the steady state friction of a glass sphere. As reported in this paper, the deconvolution of this displacement field provides a spatially resolved measurement of the actual shear stress distribution at the contact interface. First results show that the simple considerations based on actual contact area and constant shear stress hypothesis (often embedded in rough contact models) cannot account for the observed shear stress distribution. Much work remains to be done, but one of the promises of this method is the possibility of investigating local friction between patterned surfaces with well controlled topography at the micrometer level.
[A. Chateauminois and C. Fretigny, Eur. Phys. J. E 27, 221 (2008)]

June 2008
From bulk to encapsidated DNA: Energetics and density of DNA packed in bacteriophage capsids
The DNA that constitutes the genome of a bacteriophage is tightly packed in a protein shell called a capsid; this shell needs to withstand a large internal pressure from the closely packed DNA. Not much is known about the way the DNA is packed, so we have formulated a new theoretical approach to relate the density distribution of the DNA in the capsid to experimental data connecting osmotic pressure with the DNA density in the bulk. This has enabled us to determine the length of the packed DNA (packing fraction) as a function of the osmotic pressure - this is a quantity directly accessible in experiments. Somewhat surprisingly, we have found that the packing fraction can be reliably calculated even when neglecting the elastic energy of encapsidated DNA, which suggests that these experiments essentially probe the properties of the bulk DNA. Nevertheless, the elasticity of the DNA was found to influence the density distribution of the encapsidated DNA, inducing a very narrow cylindrical core that is depleted of DNA in an otherwise almost uniformly filled capsid. The radius of the depleted core (~1 nm) is small on the scale of the bacteriophage radius (~30 nm) and it diminishes with the increase of osmotic pressure. It has negligible influence on the packing fraction. We have performed packing fraction calculations for bathing solutions of different salts and concentrations. Our results, especially the predictions for MnCl2 bathing solutions, should be easily tested in experiments.
[A. Siber et al., Eur. Phys. J. E 26, 317 (2008)]

January 2008
Thermal diffusion and bending kinetics in nematic elastomer cantilever
To make an artificial muscle, we need a soft material that contracts in response to changes in its surroundings. Liquid crystal elastomers (LCE) are very special types of rubber, which combine the long range orientational order of liquid crystals and the polymer elasticity of the weakly crosslinked network to give a range of actuation properties. We have calculated the way a cantilever of this kind of material would bend if heated from one side; this is the kind of response that might be useful for applications in microfluidic valves and pumps, as well as other structures which can respond to their environment. LCE are particularly attractive as actuators because of their reversible shape change. In simple terms, a uniaxially aligned monodomain nematic LCE contracts when heated and extends back when cooled, in principle, over an infinite number of cycles [Gelling et al. J. Chem. Phys., 88, 4008 (1988); Aufhold et al. Macromol. Chem., 192, 2555 (1991); Tajbakhsh et al., Eur. Phys. J. E, 6, 181 (2001)]. To get a bending motion in a LCE cantilever, we applied a temperature gradient to generate inhomogeneous strain distribution. We modelled the dynamics of a cantilever, which is radiatively heated from one side. The cycle of induced curvature agrees with our experimental data from a range of samples and materials.
[K.K. Hon, D. Corbett and E.M. Terentjev, Eur. Phys. J. E 25, 83 (2008)]

December 2007
The sad fate of a "fakir droplet"
Hydrophobic microstructured surfaces, like the famous sacred lotus leaves, can exhibit extreme water-repellency. On such superhydrophobic surfaces, a gently deposited water droplet sits on top the highest microstructures just like a fakir on a bed of nails. The droplet keeps an almost spherical shape and can roll with as little resistance as a ball on a billiards table, making liquid deposition almost impossible. Potential applications of artificial superhydrophobic surfaces range from self-cleaning coatings for clothes or glass to drop transport on labon-a-chip devices. Unfortunately, when the microstructures pierce the drop and the liquid invades the roughness, water repellency is lost. Moreover, the impaled drop then becomes strongly pinned to the solid substrate. This impalement transition is shown to occur during the spontaneous evaporation of a water droplet, which is a major limiting factor for most of the potential applications of superhydrophobicity. The forces dictating the impalement transition were identified by monitoring the evolution of the full 3D shape of the interface below evaporating fakir droplets. This was made using model micropatterned surfaces and interference microscopy. First, it revealed a new stable wetting state halfway between the usual Fakir and impaled states. Second, it led to propose a simple model to account for all the observed impalement scenarios based on the competition between the internal drop pressure (proportional to the inverse drop radius) which pushes the drop downward and the capillary forces applied by the microststructures, which hinder the liquid penetration. Moreover, on top of the quantitative description of the experimental findings this simple picture led an efficient design strategy for "ultrarobust" water repellent coatings which should repel arbitrarily small droplets!
[S. Moulinet and B. Bartolo, Eur. Phys. J. E 24, 251 (2007)]