TRAM

Structured foams are of great interest for different material communities: metallic foams (high resistance), catalysis (high surface/volume ratio) and phononic materials (sound absorption). These foams are fabricated by the solidification of initially liquid foams. A major difficulty in manufacturing such materials is controlling the foam evolution during this first stage. This evolution is due to several phenomena: flow of the continuous phase (drainage), diffusion of gas through the liquid films, and film rupture. Recent publications show that this topic is a crucial issue for both academics and industries. One aspect of the project aims at developing an alternative to control foam drainage. The approach consists in developing a microfluidic system able to control foam drainage in order to generate highly controlled materials (bubble size and volume fraction) and being able either to reinforce or reverse drainage. The advantages of a microfluidic approach are manifold, the most important is the possibility to perform experiments within very short times (few seconds), giving the possibility to span a large range of experimental parameters in order to optimize, in a first step, drainage control. The control of foam drainage has rekindled interest in the Marangoni effect, which refers to the flows induced by a surface tension gradient that can be generated by a surfactant concentration gradient or by a temperature gradient. The objective is to generate a flow either in the direction or in the opposite direction of gravity by using temperature gradients, successively decaying/increasing the liquid fraction. The advantage of generating Marangoni flows stemming from a temperature gradient is to develop a foam drainage control which does not depend on any seeding material (magnetic particles, photo or thermo-responsive surfactant). In the reverse drainage case, the leading material will be more homogeneous, with a well-controlled liquid fraction and bubble size distribution. A challenging aspect of this approach is that a temperature gradient generates several side effects, that can be either advantageous or that need to be neutralized, provided these effects are well understood: we have shown in a recent paper [Selva et al., 2011] that a bubble undergoing a constant temperature gradient generates a flow in the surrounding liquid. The physical phenomena involved in such a system are multifaceted (thermocapillarity, solutocapillarity and potentially cell deformation for soft cell) and may have either complementary or opposite effects depending on the experimental conditions; leading to complex situations involving free interface, surfactant diffusion, heat diffusion and hydrodynamics. For this reason, the project is divided into two main steps: (i) a seeding phase at the elementary level containing a valuation; (ii) the study of foam drainage in a temperature gradient. In a first part (elementary level) we aim to rationalize the contribution of all involved effects under controlled conditions (surfactant concentration and distribution, cavity deformation, level of confinement). Our approach will be threefold: experimental, numerical and modelling legitimizing the contribution of five research complementary groups. This seeding stage will be valued by developing a system able to drive a droplet on a 2D substrate at will (controlled displacement or droplet trapping over space and time). Based on the understanding given by previous analysis, the second objective of the project is developing a study based on the previously understood effects: controlling the foam drainage, a major application in new materials perspectives. More precisely, we aim at developing an experimental set-up allowing to obtain a homogeneous foam of controlled bubble size and controlled liquid fraction stabilized for a typical time larger than the characteristic foam ageing time.