José Bico

Wetting of textured surfaces

We discuss quantitatively the wetting of a solid textured by a designed roughness. Both the hydrophilic and the hydrophobic case are described, together with possible implications for the wetting of porous materials.

Capillary Origami: Spontaneous Wrapping of a Droplet with an Elastic Sheet

The interaction between elasticity and capillarity is used to produce three-dimensional structures through the wrapping of a liquid droplet by a planar sheet. The final encapsulated 3D shape is controlled by tailoring the initial geometry of the flat membrane. Balancing interfacial energy with elastic bending energy provides a critical length scale below which encapsulation cannot occur, which is verified experimentally. This length is found to depend on the thickness as h3/2, a scaling favorable to miniaturization which suggests a new way of mass production of 3D micro- or nanoscale objects.

Slippy and sticky microtextured solids

The aim of this paper is to describe the possibility of achieving super-hydrophobic materials by tailoring their surface topography. Water droplets easily slip or roll down on such surfaces. However, it is found that microtextures on a solid can generate sticky surfaces as well, and the conditions for avoiding such an effect are discussed.

The macroscopic delamination of thin films from elastic substrates

The wrinkling and delamination of stiff thin films adhered to a polymer substrate have important applications in “flexible electronics.” The resulting periodic structures, when used for circuitry, have remarkable mechanical properties because stretching or twisting of the substrate is mostly accommodated through bending of the film, which minimizes fatigue or fracture. To date, applications in this context have used substrate patterning to create an anisotropic substrate-film adhesion energy, thereby producing a controlled array of delamination “blisters.” However, even in the absence of such patterning, blisters appear spontaneously, with a characteristic size. Here, we perform well-controlled experiments at macroscopic scales to study what sets the dimensions of these blisters in terms of the material properties and explain our results by using a combination of scaling and analytical methods. Besides pointing to a method for determining the interfacial toughness, our analysis suggests a number of design guidelines for the thin films used in flexible electronic applications. Crucially, we show that, to avoid the possibility that delamination may cause fatigue damage, the thin film thickness must be greater than a critical value, which we determine.

Wrinkling Hierarchy in Constrained Thin Sheets from Suspended Graphene to Curtains

We show that thin sheets under boundary confinement spontaneously generate a universal self-similar hierarchy of wrinkles. From simple geometry arguments and energy scalings, we develop a formalism based on wrinklons, the localized transition zone in the merging of two wrinkles, as building blocks of the global pattern. Contrary to the case of crumpled paper where elastic energy is focused, this transition is described as smooth in agreement with a recent numerical work [R. D. Schroll, E. Katifori, and B. Davidovitch, Phys. Rev. Lett. 106, 074301 (2011)]. This formalism is validated from hundreds of nanometers for graphene sheets to meters for ordinary curtains, which shows the universality of our description. We finally describe the effect of an external tension to the distribution of the wrinkles.

Self-propelling slugs

Surface coating is generally achieved by an active operation: painting with a brush; withdrawing from a bath. Porous imbibition constitutes a more passive way: a porous material just put in contact with a reservoir containing a wetting fluid is spontaneously invaded. In this case, the material can eventually be lled by the fluid. If the aim is to coat only the surface of the pores, the liquid excess must be actively removed. We present an experiment in which a liquid train spontaneously moves in a capillary tube because of the trail it leaves behind. From a practical point of view, this system achieves a coating of the tube. The condition required for this motion and a model for its dynamics are presented. We also show how these trains can drive extremely viscous liquid slugs (or even solid bodies) in narrow tubes. We discuss the thickness of the deposited lms. Extensions and limits of this system in more complex geometries are nally described, together with special cases such as the deposition of solid lms.

Ex vivo Rheology of Spider Silk

SUMMARY We investigate the rheological properties of microliter quantities of the spinning material extracted ex vivo from the major ampullate gland of a Nephila clavipes spider using two new micro-rheometric devices. A sliding plate micro-rheometer is employed to measure the steady-state shear viscosity of ∼1 μl samples of silk dope from individual biological specimens. The steady shear viscosity of the spinning solution is found to be highly shear-thinning, with a power-law index consistent with values expected for liquid crystalline solutions. Calculations show that the viscosity of the fluid decreases 10-fold as it flows through the narrow spinning canals of the spider. By contrast, measurements in a microcapillary extensional rheometer show that the transient extensional viscosity (i.e. the viscoelastic resistance to stretching) of the spinning fluid increases more than 100-fold during the spinning process. Quantifying the properties of native spinning solutions provides new guidance for adjusting the spinning processes of synthetic or genetically engineered silks to match those of the spider.

3D aggregation of wet fibers

Wet fibrous structures such as nanotube carpets or macroscopic brushes tend to self-assemble into bundles when the liquid evaporates. The aggregation process relies on a balance between capillary attraction provided by liquid bridges and restoring torque due to structure stiffness. The final self-organized structure is found to result from a cascade of pairing of smaller bundles into bigger ones. We first describe, both experimentally at a macroscopic scale and theoretically, the case of a single pair of fibers and then generalize this description to more complex 3D assemblies. We finally show the relevance of our results to micro-scale experiments from the literature.

Capillary origami controlled by an electric field

We show experimentally how an electric field can control the folding and unfolding of a thin elastic membrane around a liquid droplet. As the voltage is increased above a critical value the membrane unfolds completely. The transition is reversible, although the structure closes back for a lower critical tension. We propose scaling laws for these critical voltages based on the interaction between surface tension, elastic and electric energies. These scaling laws are in qualitative agreement with experiments and a simplified 2D simulation of the problem, providing a useful tool for designing practical micro-devices actuated by an electric field.

‘Gobbling drops’: the jetting–dripping transition in flows of polymer solutions

This paper discusses the breakup of capillary jets of dilute polymer solutions and the dynamics associated with the transition from dripping to jetting. High-speed digital video imaging reveals a new scenario of transition and breakup via periodic growth and detachment of large terminal drops. The underlying mechanism is discussed and a basic theory for the mechanism of breakup is also presented. The dynamics of the terminal drop growth and trajectory prove to be governed primarily by mass and momentum balances involving capillary, gravity and inertial forces, whilst the drop detachment event is controlled by the kinetics of the thinning process in the viscoelastic ligaments that connect the drops. This thinning process of the ligaments that are subjected to a constant axial force is driven by surface tension and resisted by the viscoelasticity of the dissolved polymeric molecules. Analysis of this transition provides a new experimental method to probe the rheological properties of solutions when minute concentrations of macromolecules have been added.