Colloids in external fields
Colloids under shear
How do crystals flow? A monolayer of nearly hard spheres sedimented in a Hele-Shaw cell and is set into motion by the flow of the supernatant colloidal fluid. The crystals flow by dislocation hopping and grain boundary switches. The surface evolves crystalline ripples.
Deionized charged spheres confined to some five layers in a plate-plate cell. With increasing shear rate (bottom to top) and/or increasing salt concentration (left to right), first registered zig-zag hopping then free gliding of layers is observed.
To obtain images which can be quantitatively interpreted, we are continuously improving known microscopy techniques and developing new ones.
The ripple crystals were observed by low aperture microscopy which constructs the images from zeroth order and incoherently scattered first order light (Langmuir 22, 1828 (2006)). The sheared crystals were observed by image correlation techniques (Proc. Roy. Chem. Soc. Faraday Disc. 123, 133 – 143 (2003)) but also Fourier microscopy is available (Prog. Colloid Polym. Sci. 118, 202 – 207 (2001))
Shear may further be used to steer crystal nucleation and growth. In the bulk shear suppresses nucleation. Once the shear is stopped, nucleation proceeds as before. Using time dependent shear multimodal crystal size distributions are obtained (J. Chem. Phys. 102, 5082-5087 (1995)).
Crystallization in confinement
Due to packing restrictions, solids in confinement show structures differing from the bulk. One often observed principle to minimize volume in hard sphere like systems is a build-up of solids from densely packed sub-units of prismatic or slab geometry (Phys. Rev. Lett. 79, 2348-2351 (1997); J. Phys.: Condens. Matter 17, S2779 - S2786 (2005)).
For charged spheres in charged parallel plate confinement these structures are missing. Instead distorted structures (e.g. rhomboedric) are expected and observed (J. Phys.: Condens. Matter 24, 464123 (9pp) (2012)). Experiments and simulations compare well in the coupling strength – reduced density diagram. Meta-stable Moire-patterns also observed in experiment are not seen in the ground state simulations.
Colloids in Gradients
Charged colloids move in concentration gradients due to diffusio-phoresis, e.g. if CO2 is allowed to enter a sample with ion exchange resin present at its bottom via a porous silicon stopper. This was theoretically described e.g. by J. L. Anderson and D. C. Prieve, Separation & Purification Reviews 13, 67-103 (1984). The resulting changes in colloid concentration may mimic an equilibrium liquid-gas phase separation. Some authors interpreted this situation incorrectly in terms of a like-charge attraction (Phys. Rev. Lett. 69, 3778-3781 (1992) and (E) ibid. 70 2823 (1993); Phys. Rev. Lett. 72, 786 and 787 (1994)).
A completely different, purely geometric gradient effect leads to the accumulation of charged particles in a charged wedge, where they form close packed structures. (J. Phys. Condensed Matter 20, 404221 (1-13) (2008)).