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?
- PhD: I focused on the flow of granular materials -solid grains in air- and the effect of inter-grain cohesive forces. I combined in situ experiments with natural snow and discrete simulations of cohesive granular flow.
- Post-doc: I am focusing on the effect of the viscous fluid which surrounds bubbles in foams, droplets in emulsions and solid grains in suspensions. I develop a new numerical method to simulate the particle deformation and motion in a viscous fluid.
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:
- steady and uniform regime in a wide range of slopes
- bi-linear velocity profile: a highly sheared basal layer and a much less sheared upper layer
- large aggregates (cm) in the flow
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.
- Plane shear flow in a periodic domain gives access to the constitutive laws as a fonction of the adjustable cohesion strength. We point out that the inter-granular cohesion strongly enhances the apparent viscosity of the materials through the formation of aggregates in the flow (Movies of granular flow with and without cohesion).
- Flows down an inclined ensure a qualitative comparison with snow experiments. In this geometry, we found that cohesive granular media, like snow, present a weakly viscous basal layer made of single grains and a highly viscous upper layer made of large aggregates.
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:
- the motion of each particle;
- particle deformation into ellispsoids through the bulk constitutive law.
Soft Dynamics is a promising tool for investigating the collective behaviors of many complex materials.
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.