Non-newtonian fluids and droplets in microchannels

Abstract

With the development of microfluidics, electro-osmotic (EO) driven flow has gained intense research interest as a result of its unique flow profile and the corresponding benefits in its application in the transportation of sensitive samples. Challenges occur when the EO driven mechanism encounters complex rheology and vital questions such as Can the zeta potential still be assumed to be constant when dealing with fluids with complex rheology?, Does the shear thinning effect enhances electroosmotic driven flow? need to be answered. Experiments were conducted via using current monitoring and microscopy fluorescent methods, and a analytical model was developed by coupling a generalized Smoluchowski approach with the power-law constitutive model. The zeta potential was calculated. The shear thinning effect is also addressed via experimental data and theoretical calculations. The mathematical model for the two immiscible layers of electro-osmotic driven flow in the parallel microchannel was proposed. One layer is a conducting nonNewtonian power-law fluid driven by electro-osmotic force. The other layer is a nonconducting Newtonian layer driven by interface shear. The effects of Debye-Hueckel parameter xhi, interfacial zeta potential If/I , the Newtonian viscosity 1'2' the non-Newtonian fluid consistency coefficient m & flow behavior index n were discussed. The complex flow behavior, namely fluid consistent coefficient and flow behavior index, play important roles in the velocity distributions. The shear thinning effect is also analyzed. The results show that the shear thinning fluid is not only ideal for direct electro-osmotic driving but also for hybrid driving. A flow-focusing geometry in a microfluidic device was studied for the formation of uniform droplets and we qualitatively illustrated aspects of controlling the droplet size and breakup regimes when an active electric field is applied. The control of droplet size was demonstrated with applied electric fields by changing the voltage and frequency. Various droplet breakup regimes including squeezing, dripping, unstable breakup and jetting induced under different electric field parameters were observed. It is shown that the droplet size decreases with an increase in voltage. Similar decreasing of the droplet size is also found with the increase of electric field frequency, especially when the frequency IS less than 2 kHz. In addition, the experimental results show the droplet size IS much more III uniform at a lower frequency than that at a higher frequency. Flow focusing microchannels with three orifice sizes and the non-contact type of electrodes were designed and fabricated for investigations of non-Newtonian droplet formation under the influence of applied AC electric fields. Non-Newtonian fluids that have similar rheological behavior of bio samples were adopted for droplet formation. Flow conditions of experiments, microchannel geometries, and AC electric field parameters have been implemented systematically. The influences of these variables were analyzed.…