Flow and heat transfer in microfluidic devices with application to optothermal analyte preconcentration and manipulation

This work describes a novel optothermal method for electrokinetic concentration and manipulation of charged analytes using light energy, for the first time. The method uses the optical field control provided by a digital projector to regulate the local fluid temperature in microfluidics. Thermal characteristics of the heating system have been assessed by using the temperature-dependent fluorescent dye method. Temperature rises up to 20 C (maximum temperature achieved in this experiment was about 50 C) have been obtained with the rate of about 0.8 C/s. The effect of the source size and light intensity on the temperature profile is investigated and the ability of the system to generate a moving heat source is demonstrated. A theoretical investigation is also performed by modeling the system as a moving plane source on a half-space. Effects of heat source geometry, speed, and power on the maximum temperature are investigated and it has been shown that by choosing an appropriate length scale, maximum temperature in dimensionless form becomes a weak function of source geometry. For the flow field control in the proposed system, the fundamental problem of fluid flow through straight/variable cross-section microchannels with general cross-sectional shapes are investigated. Approximate models are developed and verifications are performed by careful independent experiments and numerical simulations. Further verification is also performed by comparing the results with those collected from the literature. The concentration enrichment in the present approach is achieved by balancing the bulk flow (either electroosmotic, pressure driven, or both) in a microcapillary against the electrophoretic migrative flux of an analyte along a controlled temperature profile provided by the contactless heating method. Almost a 500-fold increase in the local concentration of sample analytes within 15 minutes is demonstrated. Optically-controlled transport of the focused band was successfully demonstrated by moving the heater image with the velocity of about 167μm/min. Transporting the concentrated band has been achieved by adjusting the heater image in an external computer. This ability of the system can be used for sequential concentration and separation of different analytes and transporting the focused bands to the point of analysis.