Transient rheology of solvent-responsive complex fluids by integrating microrheology and microfluidics

A new microrheology set-up which allows us to quantitatively measure the transient rheological properties of solvent-responsive complex fluids was constructed by integrating particle tracking microrheology and microfluidics. The dialysis cell consists of a reservoir, porous dialysis membrane, and sample chamber. Solvent molecules can freely diffuse between the reservoir and the sample chamber while macromolecular sample components are trapped in the sample chamber with a rigid semipermeable dialysis membrane. The design enables manipulation of the solvent composition in the sample chamber by simply switching the fluid composition in the reservoir. Validation experiments for solvent diffusion in the dialysis cell showed good agreement with numerical solutions of the transport equations and confirmed that the solvent composition in a sample can be changed in a controlled and predictable fashion within a few minutes due to the small device dimensions. For aqueous solutions of sodium alginate and sodium polystyrene sulfonate, transient rheological properties were measured in response to changes in salt concentration. The results showed that the dialysis cell is a useful and versatile tool to study the response of complex fluids to reversible changes in solvent composition. (c) 2006 The Society of Rheology.