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.

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)]