Research interests: Behavior of multiphasic materials

Overview

I am interested in the granular materials, emulsions and foams. Do they behave like an elastic solid, like a viscous fluid? What is the role of their internal structure?

Snow Flow

How does snow flow? Like water? Like mud? Like sand? This question is crucial to predict which areas are exposed to avalanches, and how efficient defense structures will be. However, experiment with snow are difficult and therefore scarce. Due to the lack of data, the behavior of natural snow is not fully described yet.
In my PhD, we performed extensive experiments with natural snow flowing down a 10 m long flume. The specificity of this experiment is that they were performed in situ to get access to natural snow (see photos of the experimental test-site). From hundred runs at adjustable slopes and flow rates, we extract some generic characterisics of snow flow which differ from those of common threshold fluids:

Snow flows present some similarities with flow of dry grains (sand). This is not astonishing since snow is made of small ice grains (diameter 0.2 mm). However, the snow velocity profile, which displays two distinct layers, differs from that of a dry granular flow. This difference can be interpreted as a consequence of the presence of large aggregates resulting from inter-grain cohesive forces.



Cohesive granular flow

Many granular materials present significant inter-grain cohesive forces (powders, wet sand, snow...). However, their role on the rheological behavior is still largely ignored. In my PhD, I investigated cohesive granular flow through Molechular Dynamics simulations. In this method, the motion of each grain is derived from Newton's laws, according to the contact forces with its neighbors.



Soft Dynamics simulation

Materials made of soft and/or concentrated units in a liquid, such as emulsions, foams, vesicles, dense suspensions, exhibit unexpected behaviors which are not fully described yet. Is there a common constitutive law which describes this kind of materials? How can dilatancy affect their behavior? What are the conditions of shear-banding? Discrete simulation methods are helpful in such investigations for either dry granular materials or dilute suspensions.
In my Post-doc I develop a new, generic simulation method called Soft Dynamics. It accurately describes the interaction of close-packed units in a viscous fluid, such as in foams, emulsions and dense suspensions. The difficulty is that the elastic deflection of the particle surface and the viscous flow in the gap are strongly intertwined. As a function of their interaction via the viscous fluid, Soft Dynamics describes:

Soft Dynamics is a promising tool for investigating the collective behaviors of many complex materials.

Overview

Soft Dynamics: sketch of normal particle interaction.
A force between elastic surfaces (modulus E) is transmitted partly through the fluid (visosity η) and partly through possible remote (e.g. van der Waals) interaction (labeled W). It can deform them (deflection δ), which in turn affects the flow. The consequence is that the center-to-center distance X and the gap h between both surfaces exhibit two distinct dynamics.