P. G. de Gennes’s research while affiliated with Collège de France and other places

We study the kinetics of specific adhesion of giant vesicles on complemetary (bilayer-decorated) solid surfaces. Tensed vesicles exhibit a single adhesion zone that grows slowly. Floppy vesicles adhere via many small adhesion spots that grow and fuse, merging finally into a large adhesive zone. Using approaches derived from Ref. 12, we show how the progressive mobilization of adhesive molecules (diffusing toward the patch) can explain our experimental observations.

A non adsorbing, flexible polymer (in dilute solution with a good solvent)enters a pore (of diameter D smaller than its natural size R F only when it is sucked in by a solvent flux J higher than a threshold value J c . For linear polymers $J_c \simeq kT/\eta$ (whereT is the temperature and η the solvent viscosity). We discuss here the case of statistically branched polymers, with an average number n of monomers between branch points. We find that, depending on the tube diameter, there are two regimes: “weak confinement” and “strong confinement” depending on the tube diameter. By measuring J c in both regimes, we should determine both the molecular weight of the polymer and the number n of monomers in it.

First of all I am immensely pleased to be here. I have had great interest in India over many years. I want to remember that we are standing in the Science Museum and the aim of this Science Museum is to make science stimulating and attractive to young minds. Also to older minds. Bubbles have inspired people for centuries. I think this is an interesting subject. I think also that education in recent times will be ultimately useful and I hope to show this to you at the end of this talk. Let me start by saying what a bubble really is. It is a thin sheet of water and to this water we must add a little soap. That is very critical and I will try to give you a feeling why we need the soap. These things have color, these things are mobile. Not only the whole globule will move around but if you take your John Boy and you have him look at it closely, you will notice that on the surface, there are all strange movements forming and deforming and ultimately it has a short life. So, where is the life for the bubble, where is color for the bubble – I want to talk about all of these.

The aim of the present paper is mainly tutorial. The original set up is an incompressible liquid thread, originally horizontal, and attached at both ends. Under its own weight, the thread sags and thins. A complete theoretical discussion was carried out three years ago by Teichman and Mahadevan —using rather complex mathematical tools. We show here how the main results can be obtained very simply. A single characteristic time td emerges. At ttd the distortions is weak, with a fall amplitude growing like . At later times, the stretching is strong, and the stretched length diverges at a time ~td. Extensions of these arguments are also quoted for various other cases: spinning filaments, filaments in a flowing fluid bath ...

This paper is a tutorial description of stick-slip in soft materials (rubber beads, gels) where inertial effects are negligible. A typical example is a rubber sphere, pressed against a glass surface (JKR contact). The sphere is driven from the top at a prescribed velocity U ( approximately 100 microm/s). At moderate U (and with suitable surface treatment) a periodic stick-slip regime is often observed. We present a simple picture of the stick-slip cycle, describing the growth of a slip zone from the rear end of the sample, and the resulting plot of force vs. time. All our estimates are restricted to scaling laws.

In several glassy polymer-solvent systems the observed initial uptake of solvent is linear in time. A sharp front moving at a constant velocity v0 divides the polymer into two regions: ahead of the front the solvent concentration is low and the polymer is still glassy, while behind it the concentration is high and the polymer has been plasticized. Here a simple description of this process is presented. It is based on the idea that the slow plasticization kinetics sets an upper limit for the flux of solvent into the glassy region. Our description predicts that the front velocity will crossover to the usual 1/√t behaviour at t ≈ τ1 = D1/v02 (here D1 is the diffusion coefficient for solvent in the plasticized region) and that an induction time (whose duration depends on the value of the diffusion coefficient D0 in the glassy region) will precede the establishment of the front.

We analyse an imbibition process where a water/oil interface moves in a porous, hydrophilic structure with a macroscopic velocity V. Our microscopic description is based on the classical interpretation of the Hoffmann-Tanner law, where the local contact line velocity VL is proportional to the cube of the dynamic contact angle θ, and the dissipation (per unit length of line) scales like γθ2VL (where γ is the water/oil interfacial tension). The result is a dynamic capillary pressure Δpd ~ γ/rt (Ca)2/3 τ5/3, where τ is a tortuosity factor (>1) and rt a throat size. The data of Weitz et al. give Δpd ~ Ca0.5±0.1 and could be interpreted in this way.

The scattering intensity, from colloidal grains carrying a diffuse adsorbed layer, contains two components: one ( (q)) is due to the average concentration profile, and has been considered by the British school; the other component (q) is due to fluctuations in the adsorbed layer. For neutral, flexible, chains, adsorbed from a good solvent, the adsorbed layer is self-similar and should show strong fluctuations. We construct scaling formulae for and in the most relevant regime, where q-1 is larger than a monomer size a, but smaller than a coil size R. For matched grains (no contrast between solvent and grain) the fluctuation effects are sizeable, and scattering is not a very good probe of the inner structure of the adsorbed layer. With mismatched grains, the fluctuation effects are relatively unimportant, and the structure of the average profile should show up as a correction to the Porod law, proportional to q-8/3.