Microfluidic shear stress measurement : liquid crystal technique

Preliminary work towards developing a liquid crystal based technique for direct shear stress measurements over fluid / solid surfaces in microfluidic systems is presented. The particular previous work in microfluidics which motivated the development of this technique is presented, as is general background on microfluidics. The theory of shear sensitive liquid crystals is reviewed and then expanded upon in regards to the specific type of flow considered in this study; slug flow. Slug flow is a regime of two phase flow which can occur when two immiscible fluids are pushed through the same microchannel. Under the correct conditions the two fluids will separate into well defined alternating slugs which span the entire channel. The shear stress field near the interface between the fluids is not currently well understood, which is the problem that provides the motivation for this study. A prototype apparatus is described which is capable of generating slugs, and has appropriate optical access to test the liquid crystal shear stress measurement technique. The microchannel (150um x 250um laser etched glass), auxiliary flow (Cole-Parmer Infusion 100 Syringe Pump), and optical data collection (Olympus BX51 microscope) subsystems are all described in detail. Procedures for applying the cholesteric liquid crystal mixtures obtained through Pressure Chemicals Ltd. to the microchannel and for collecting liquid crystal data are described as well. Finally, preliminary results are presented, the current status of the technique is stated along with proposed directions for future research work