Abstract Freezing of water flowing through a small channel can be used as a nonintrusive flow control mechanism for microfluidic devices. However, such ice valves have longer response times compared to conventional microvalves. To control and reduce the response time, it is crucial to understand the factors that affect the flow freezing process inside the channel. This study investigates freezing in pressure-driven water flow through a glass channel of 500 μm inner diameter using measurements of external channel wall temperature and flow rate synchronized with high-speed visualization. The effect of flow rate on the freezing process is investigated in terms of the external wall temperature, the growth duration of different ice modes, and the channel closing time. Freezing initiates as a thin layer of ice dendrites that grows along the inner wall and partially blocks the channel, followed by the formation and inward growth of a solid annular ice layer that leads to complete flow blockage and ultimate channel closure. A simplified analytical model is developed to determine the factors that govern the annular ice growth, and hence the channel closing time. For a given channel, the model predicts that the annular ice growth is driven purely by conduction due to the temperature difference between the outer channel wall and the equilibrium ice-water interface. The flow rate affects the initial temperature difference, and thereby has an indirect effect on the annular ice growth. Higher flow rates require a lower wall temperature to initiate ice nucleation and result in faster annular ice growth (and shorter closing times) than at lower flow rates. This study provides new insights into the freezing process in small channels and identifies the key factors governing the channel closing time at these small length scales commonly encountered in microfluidic ice valve applications.

中文翻译：

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小圆柱通道中流动冻结过程中的冰形成模式

摘要 流经小通道的水的冷冻可用作微流体装置的非侵入式流量控制机制。然而，与传统微型阀相比，这种冰阀具有更长的响应时间。为了控制和减少响应时间，了解影响通道内流动冻结过程的因素至关重要。本研究使用与高速可视化同步的外部通道壁温度和流速的测量值来研究压力驱动的水流通过 500 μm 内径的玻璃通道时的冻结。从外壁温度、不同冰模式的生长持续时间和通道关闭时间方面研究了流速对冷冻过程的影响。冻结开始为沿内壁生长并部分阻塞通道的薄冰枝晶层，随后形成并向内生长实心环形冰层，导致完全流动阻塞和最终通道关闭。开发了一个简化的分析模型来确定控制环形冰生长的因素，从而确定通道关闭时间。对于给定的通道，该模型预测环形冰的生长完全由传导驱动，这是由于通道外壁和平衡冰水界面之间的温差。流速影响初始温差，从而对环形冰的生长产生间接影响。较高的流速需要较低的壁温来启动冰成核，并且与较低的流速相比会导致更快的环形冰生长（和更短的闭合时间）。这项研究为小通道中的冷冻过程提供了新的见解，并确定了在微流体冰阀应用中常见的这些小长度尺度上控制通道关闭时间的关键因素。