Non-newtonian fluids and droplets in microchannels
Date of Issue2016
School of Mechanical and Aerospace Engineering
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. Among them, the non-Newtonian droplet formation was highlighted and addressed. The dependency of the flow condition and electric field on the non-Newtonian droplet formation dynamics was presented and analyzed. The flow field of the non-Newtonian droplet formation was measured and analyzed quantitatively via a high speed /lPIV system. Different droplet formation regimes and the impact of AC electric field were considered when the measurements were conducted. Flow fields and the related vorticity distributions were used for flow characterization. A particle free method for flow field visualization was proposed and achieved by analyzing liquid crystal polarization. The proposed concept is implemented by imaging liquid crystal flow under microfluidic environment usmg a polarization based optical interferometric configuration. Fringe patterns give good presentation of flow characterizations for different nozzle/diffuser microchannel designs. The obtained results demonstrate that the flow shear and flow recirculation under various condi tions can be evaluated in terms of interferometric fringe patterns. It is envisaged that the proposed methodology can make a potential impact in flow field visualization studies and related analysis. A novel method for the investigation of the dynamics of droplet formation was proposed by liquid crystal polarization as the traditionally adopted ~PIV method had its vulnerability in interfacial and filament measurement. The interfacial dynamics of the droplet and the filament were observed and the associated flow characteristics were analyzed. In addition to the formation dynamics, the control of liquid crystal droplet generated in flow focusing micro channel was achieved by hydrodynamic alteration and implementation of AC electric field. The exponential decrease in liquid crystal droplet size in terms of capillary number was found. Micro level (urn) of droplet size adjustment was obtained in the presence of AC electric field in a microfluidic environment.