I am an experimental physicist, working on materials that are known under a few different names, each reflecting part of their behaviour. Soft matter indicates the materials are soft (at least compared to metals and rocks) under applied stresses and forces. Disordered matter indicates the absence of crystalline order in the particles that make up the material, while complex fluids indicates these materials can flow, and when they do they flow in a complex manner vastly different from e.g., water.

The soft, disordered, complex fluids I study are foams (shaving cream) and emulsions (mayonaise), which consist of bubbles/droplets dispersed in a fluid. I am most interested in relating the properties and dynamics at the bubble/droplet level to the intriguing global behaviour of these materials, which can bear finite stresses, like a solid, but which will flow, once these stresses exceed a critical value.

The Flow of Foams

This movie shows a top view of a two-dimensional foam layer that is sheared linearly by the moving boundaries at the top and the bottom. We have succeeded to relate the drag forces acting at the bubble level to thw global average velocity profiles. In cylindrical shear, the situation turns out to be more complicated, with non-local effects altering the bulk rheology.

Static foams: critical scaling near Jamming

We prepare distinct static packings of foam bubbles. By varying the packing fraction we can approach the jamming transition as a function of density, see the sequence of 4 experimental images at the right. By advanced image analysis and rheometrical measurements we can then extract various quantities that signal critical behaviour near the jamming transition, such as the contact number Z and the shear and the bulk modulus. Our results show that a foam exhibits a jamming transition at a packing fraction f = 0.84, as shown in the lower plot, where the average contact number decreases to 4, the value at jamming, as    Z-4 ~( f-0.84)1/2.

Emulsion Rheology

We study emulsions by fast 3D confocal microscopy. We find the droplets' centers of mass by using the Sphere Matching Method, as pioneered by Brujic et al. The movie below shows first results from this method on a 60x60x60 micron scan of a polydisperse decane in water/glycerol emulsion.