Mechanical Properties of a Colloidal Gel by Depletion Interaction

$\ 1. How do we induce depletion interaction? We disperse bigger colloidal particles in the sea of small particles that are usually micelles or polymers. When big particles get close, a depletion volume where the smaller particles cannot access is formed between two bigger particles. Due to osmotic pressure, the bigger particles are pushed together by smaller particles. The potential depth is proportional to the depletion volume and the number concentration of the small particles. The potential width is the same as the diameter of the small particle. Other semidilute colloidal systems also form networks with depletion interaction. 2. How do we measure the mechanical properties of a colloidal gel? Compressional modulus can be measured from the steady state volume fraction profile of a colloidal gel [1], while shear modulus can be measured from any commercial rheometer. The basic idea of measuring compressional modulus is finding a relationship between the applied stress and volume fraction. Colloidal gels are compressed, not sheared, during creaming and their volume fractions keep changing with time. So we define a compressional modulus as a function of volume fraction such that (1) Figure below shows the gravitational stress at the top of the emulsion and volume fraction. The stress is simply buyoyancy stress by whole emulsion and volume fraction can be measured from skimped emulsion from the top of emulsion. Now, we find the relationship between stress and volume fraction from the fitting with (2) , where is a stiffness parameter that depends on the depletion interaction. From eq. (1) and (2), the compressional modulus can be estimated as below. (3) From the compressional modulus we can even predict volume fraction profiles and final heights of the cream layer. 3. How does compressional modulus or stiffness parameter, α , depend on the depletion interaction? We find that the stiffness parameter, α , depends linearly on the depletion energy and inversely proportional to the effective range of depletion interaction and the cross-sectional area of a colloidal particle; 4. How about Shear Modulus? We find that the plateau modulus, G'p, increases strongly with the volume fraction, and that is proportional to the concentration of depletant. [1] C. Kim, et. al, PRL, 99, 028303 (2007) Do you have any comments or suggestions? E-mail me! Page maintained by   Chanjoong Kim Department of Physics and DEAS Harvard University 9 Oxford St. McKay 518  Cambridge, MA 02138  617-384-9506  ckim@deas.harvard.edu$ shapeimage_2_link_0