(535j) Homogenization of Self-Organizing Colloidal Particles in a Microscopy Chamber

Colloidal particles are typically observed to self assemble in a thin chamber under a compound light microscope. It is desirable to initiate observations with a homogeneous suspension of particles. There are two ways of homogenizing or “mixing; one is “chaotic”, and the other is “separation and recombination” or SAR, which forms the basis of in-line static mixers functioning at low Reynolds’ number. Another term applied to this method is “interfacial surface generation” (ISG), which is a good description of what actually happens. Most electroosmosis based mixers in microfluidics tend to be of the chaotic type, introducing localized regions of backflow at the low Reynolds’ numbers that characterize microfluidics. In our mixing module the objective is actually to separate particles that had once reacted with one another via long-range forces so that a self-assembly experiment can be repeated (at a modified temperature, for example). It is assumed that any non-uniform fluid motion will break up the investigator’s colloidal assembly. In order to achieve the generation of new interfacial surfaces small finite elements of a static fluid need to be moved with respect to each other. Electroosmosis accomplishes this by the application of an electric field in the y direction causing a reactive force in the diffuse double layer of the chamber wall if the wall is charged and there are ions in solution. Usually the wall carries fixed negative charges and the fluid adjacent to the wall drifts toward the cathode. In a closed chamber this flow is balanced by flow in the opposite direction along the midplane of the chamber. With the Smoluchowski slip velocity at the wall given by v_y = μ_eoE and assuming we want an element of fluid at the wall to move ½ the length of a 2 cm chamber in 10s to get v_y = 0.1 cm/s we will need a potential of the order of 100 V based on the generous assumption of electroosmotic mobility μ_eo = 0.001 cm²/V-s. For the desired mixing effect we should repeat this process so that each element of fluid is brought to a different location with every repetition. Thus a pulse train starting with 100V in one direction for 10 s followed by -100V for a shorter time followed by multiple subsequent alternating square waves of progressively shorter duration should produce satisfactory micromixing in 2-3 minutes. This process was implemented in a 0.7 x 7.0 x 40 mm glass chamber with microscope observation. Video images demonstrated that all particles in the viewing field were relocated. This research was supported by NASA contract NNX12CE76P.