The effect of channel diameter on flow freezing in microchannels

Abstract

An understanding of the factors that affect the flow freezing process in microchannels is important in the development of microfluidic ice valves featuring well-controlled and fast response times. This study explores the effect of channel diameter on the flow freezing process and the time to achieve channel closure. The freezing process is experimentally investigated for a pressure-driven water flow (0.3 ml/min) through three glass microchannels with inner diameters of 500 μm, 300 μm, and 100 μm, respectively, using channel-wall temperature measurements synchronized with high-magnification, high-speed imaging. Freezing invariably initiates in supercooled water as a thin layer of dendritic ice that grows along the inner channel wall, followed by the formation and growth of a thick annular ice layer which ultimately causes complete channel closure. The growth time of the annular ice layer decreases monotonically with channel diameter, with the 100 μm channel having the shortest closing time. Specifically, the mean closing time for this smallest channel is measured to be 0.25 s, which is markedly shorter compared to other reports in the existing literature using larger channel sizes at similar flow rates. A model-based analysis of the freezing process is used to show that the total latent heat released by the freezing mass (which varies as the square of the channel diameter) is the key factor governing the closing time. Owing to this simple scaling, the study reveals that reducing the channel diameter offers an attractive approach to increasing the responsiveness of ice valves to achieve non-intrusive flow control at high sample flow rates.