New Developments in Colloid Science
Original entry: Xu Zhang, APPHY 225, Fall 2009
New Developments in Colloid Science D.A. Weitz and W.B. Russel, MRS Bull. Feb 2004 83 (2004)
colloids, colloidal materials, "directed" assembly, rheology, suspensions
This article summarizes several new developments of colloid science in two aspects: "directed" assembly of colloidal particles and understanding in rheology of colloidal suspensions.
What is colloid?
- Colloids are small solid particles dispersed in a medium, typically a fluid. This area has a long history of development, extending from the [ink making ]to the photonic-bandgap materials being developed today.
- 1) Using depletion interaction
- Controllable elements:
- 1.surface treatment
- 2.solvent choice
- 3.addition of other materials
- 2) Applying additional stresses ( Van Blaaderen )
- Ex. a hard wall that restricts the states of the colloidal suspension, electric and magnetic fields, surface stresses induced by drying, oder imposed by patterned surfaces, gravitational effects,etc.
- 3)Using interactions between colloidal particle
- Because thermal energy allows particles to explore phase space and achieve the lowest-energy structure, the thermalization of the particles in response to Brownian motion is essential to allowing them to self-assemble. Because the interparticle interaction energies are often far greater than <math>k_B T</math>, attractive interactions will drive the particles together to form local clusters, and these clusters can span space to form connected networks or colloidal gels.
- The surface energy drives the interface of the particle to be spherical, which places fundamental limitations on the structures that can be built.An important limitation comes in making photonic-bandgap structures
- 1)Mix particles of different sizes and create binary alloy crystal structures
- 2)Encapsulate spherical particles in emulsion droplets that are then collapsed by drying (https://sites.google.com/a/manoharan.deas.harvard.edu/www/publications V.N. Manoharan, M.T. Elsesser, and D.J. Pine, “Dense Packing and Symmetry in Small Clusters of Microspheres”, Science 301: 483–487 (2003))
- Although dispersions generally flow easily at moderate solids fractions, the viscosity at low stresses necessarily diverges near transitions to the solidlike phases. Wagner and Bender apply a large stress,viscous forces can overwhelm the thermodynamic or interparticle forces and push the particles close together.The understanding of this interparticle interaction is very useful in enabling full technological exploitation of concentrated colloidal suspensions.