Manipulation of Microfluidic Flows Using Time Dependent Wall Temperatures

One module in a bioagent detector currently under development involves a flow-through PCR module [1] [3] [4]. Conventional, flow-through PCR devices utilize three heaters to obtain the required temperatures in each zone, the length of which is specified by the required sample residence times. An alternate design uses two wall heaters with substrate conduction supplying the center zone temperature. The concept of using a conduction based PCR device led to an extensive computational study of various channel wall temperature profiles that would produce enhanced mixing in a variety of microfluidic devices. The results are applicable to micro channel designs in general even tough motivated by the conduction based PCR configuration. The lattice Boltzmann (LB) method was used to perform low Reynolds number (typically Re=0.10) simulations for two and three dimensional channel geometries having various wall temperature distributions. The momentum and thermal lattice Boltzmann equations were coupled via a body force term in the momentum equation. Initial computations using two- and three-heater configurations in two dimensions demonstrated excellent comparisons with published data provided that both the top and bottom walls were heated. If only one wall was heated, large vertical thermal gradients occurred resulting in non-uniform temperature fields. However, when the same conditions were applied to three dimensional channels, lower temperatures were observed in the center of the channel regardless of the wall temperatures or channel aspect ratio. Parametric studies were performed to evaluate the effects of thermal coupling, thermal diffusion coefficients, entrance temperatures, wall temperature configurations and channel geometry. If was found that moderate variation of the thermal diffusion coefficient produced only minor differences in the temperature field, and large changes in the thermal coupling magnitude demonstrated transition from natural to forced convection flows. The simulations also indicate that the largest effect on flow and temperature uniformity arises from the applied wall temperature distribution (various thickness channel walls). It was found, in 2D, that if the channel wall starts from ambient temperature, the applied heating, on the outer surfaces only, may not result in the desired wall or fluid temperatures. However, once the channel walls are heated to a uniform temperature, excellent temperature distributions are obtained for both thick and thin channel walls. Additionally, a checkerboard pattern of wall heaters was used to test its application to promoting mixing. Results were favorable in creating enhanced mixing; however, the temperature pattern did not produce uniform temperature profiles in the channel.