A. Buguin

Bouncing or sticky droplets: Impalement transitions on superhydrophobic micropatterned surfaces

When a liquid drop impinges a hydrophobic rough surface it can either bounce off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel droplets). The analysis of drop impact and quasi-static loading experiments on model microfabricated surfaces allows to clearly identify the forces hindering the impalement transitions. A simple semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir non-wetting states. Motivated by potential applications in microfluidics and in the fabrication of self-cleaning surfaces, we finally propose some guidelines to design robust superhydrophobic surfaces.

Traction forces exerted through N-cadherin contacts

Background information. Mechanical forces play an important role in the organization, growth and function of living tissues. The ability of cells to transduce mechanical signals is governed by two types of microscale structures: focal adhesions, which link cells to the extracellular matrix, and adherens junctions, which link adjacent cells through cadherins. Although many studies have examined forces induced by focal adhesions, there is little known about the role of adherens junctions in force‐regulation processes. The present study focuses on the determination of force transduction through cadherins at a single cell level.

Interplay of RhoA and mechanical forces in collective cell migration driven by leader cells

The leading front of a collectively migrating epithelium often destabilizes into multicellular migration fingers where a cell initially similar to the others becomes a leader cell while its neighbours do not alter. The determinants of these leader cells include mechanical and biochemical cues, often under the control of small GTPases. However, an accurate dynamic cartography of both mechanical and biochemical activities remains to be established. Here, by mapping the mechanical traction forces exerted on the surface by MDCK migration fingers, we show that these structures are mechanical global entities with the leader cells exerting a large traction force. Moreover, the spatial distribution of RhoA differential activity at the basal plane strikingly mirrors this force cartography. We propose that RhoA controls the development of these fingers through mechanical cues: the leader cell drags the structure and the peripheral pluricellular acto-myosin cable prevents the initiation of new leader cells.

Mathematical Description of Bacterial Traveling Pulses

The Keller-Segel system has been widely proposed as a model for bacterial waves driven by chemotactic processes. Current experiments on Escherichia coli have shown the precise structure of traveling pulses. We present here an alternative mathematical description of traveling pulses at the macroscopic scale. This modeling task is complemented with numerical simulations in accordance with the experimental observations. Our model is derived from an accurate kinetic description of the mesoscopic run-and-tumble process performed by bacteria. This can account for recent experimental observations with E. coli. Qualitative agreements include the asymmetry of the pulse and transition in the collective behaviour (clustered motion versus dispersion). In addition, we can capture quantitatively the traveling speed of the pulse as well as its characteristic length. This work opens several experimental and theoretical perspectives since coefficients at the macroscopic level are derived from considerations at the cellular scale. For instance, the particular response of a single cell to chemical cues turns out to have a strong effect on collective motion. Furthermore, the bottom-up scaling allows us to perform preliminary mathematical analysis and write efficient numerical schemes. This model is intended as a predictive tool for the investigation of bacterial collective motion.

Active atomic force microscopy cantilevers for imaging in liquids

We make an atomic force microscopy (AFM) cantilever oscillate by having an ac current circulating along it. When interacting with a permanent magnet, a normal force acts on the magnetic loop formed this way and induces its vibration. By choosing the current frequency at the resonance of the spring, this effect can be used for imaging in fluids in intermittent contact mode as demonstrated by an image of actin filaments in buffer. These active cantilevers can also be used in dc mode to reach a static position; they should be suitable for a parallel control of multiple probes.

Bursting of a Liquid Film on a Liquid Substrate

We study metastable PolyDimethylSiloxane (PDMS) films deposited on a substrate of fluorinated PDMS: after nucleation, a hole grows at constant velocity V. V is inversely proportional to the substrate viscosity and is proportional to - S, where S(< 0) is the spreading parameter. The velocity is a decreasing function of the film thickness e; the detailed V(e) law can be interpreted by a balance between capillary forces and viscous forces acting on an almost flat rim.

Adhesion on Microstructured Surfaces

Using a homemade setup, we investigated the adhesion between soft elastic substrates bearing surface microstructures (array of caps [resp., holes] of height [resp., depth] h) and a smooth surface of the same rubber. In the framework of the classical model developed by Johnson, Kendall, and Roberts, we show the following. (i) The existence of a critical height h c for the microstructures, resulting from a competition between the adhesion energy and the elastic deformation energy necessary to invade the pattern: for h < h c , the bead and the substrate are in intimate contact even when the applied force is zero, and for h > h c , an air film remains intercalated in the microstructure, and the contact is limited to the top of the caps or between the holes. The transition between these two states can be induced by increasing the squeezing force. (ii) The adhesion energy, W, of intimate contacts (h < h c ) decreases as the height increases. Suspended contacts correspond to a low adhesion and a nearly Hertzian behavior. Using simple scaling arguments and a two-level energetic description (single microstructure and whole contact) we propose a semiquantitative description of these observations.

Microfabricated arrays of elastomeric posts to study cellular mechanics

We present an approach to fabricate an array of elastomer posts in order to dynamically measure the traction forces exerted by living cells on a surface with a micrometer lateral resolution. Arrays of closely spaced vertical microposts are made in silicone elastomer [poly(dimethylsiloxane) (PDMS)] by molding a Silicon substrate that has been machined by deep Si etching after standard photolithography. The surface of the micropillars was modified to allow cell culture. Deflections of the calibrated posts were dynamically followed by direct obervation with an optical microscope. By using this set-up, we could dynamically draw up a cartography of the local traction forces exerted by the cells.

Wet Versus Dry Friction

We study the dynamics of adhesion of soft objects (rubber bead, vesicles, living cells) on wet substrates by dewetting of the intercalated films, and the friction when the substrate moves at velocity U. Wetting transition are observed above a threshold velocity, leading to a lost of adhesion and a strong decrease of friction. It corresponds to the aquaplaning for cars (or people) stopping on a wet soil.© 2005 ASME