Using a Biaxial Confocal-Rheoscope to Study Squishy Materials
Fascinating rheological properties like shear thickening/thinning and anisotropic viscosity arise from underlying structure in complex fluids. We develop and use techniques to simultaneously analyze emergent, large-scale properties and image particle-level positions and stresses in such suspensions.
Here a colloidal crystal freezes onto a square lattice template. The video is sped up by a factor of about 20.
This movie shows the break-up of a lyotropic surfactant in the lamellar phase. The break-up is symmetrip about the minimum radius and no satellite drops are formed. The frame rate is 20,000 f.p.s.
Bilayer colloidal crystal composed of ~1 micron silica colloids with a sticky "depletion" interaction. The colloidal particles that are not part of the crystal can be seen diffusing around.
Colloidal silica dimers undergoing rotational and translational Brownian motion.
The shear cell used for most of the colloid experiments.
We work in collaboration with the Procter & Gamble Company to understand the flow and breakup of complex fluids. In particular, we study the pinch-off dynamics of fluids (e.g. lyotropic surfactants and thermotropic liquid crystals) possessing liquid crystalline order.
Why do some materials grow near-perfect crystals with mirror-smooth faces whereas others grow rough, bumpy crystals? Our group has recently gotten a glimpse of crystal growth in real time — not by watching individual atoms, but rather by freezing model atoms that can be observed directly with an optical microscope.
Colloidal suspensions – where micro-size or nano-size particles are suspended in a fluid – exhibit various equilibrium structures ranging from face-centered and cubic-centered crystals to binary ionic crystals, and even kagome lattices. When driven out-of-equilibrium by shear, even more diverse colloidal structures can be accessed. These structures lead to unique flow behaviors of suspensions.
In thermal equilibrium, particles suspended in a fluid randomly move about due to kicks from the fluid molecules, in what is known as Brownian motion or diffusion. Shear a fluid, however, and the particles' diffusion will be greatly enhanced. Why? Diffusion spreads some of the particles to regions of the fluid with different velocities. As the fluid then carries different particles with different speeds, the particles spread out faster, effectively increasing the diffusion. This mechanism, dubbed Taylor dispersion after its discoverer G. I.