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Highlights • Flow freezing is studied in a small channel, suitable for microfluidic ice valves. • Ice formation modes are identified during flow freezing: dendritic and annular. • High-speed imaging is used to characterize the ice formation modes. • The effect of flow rate on freezing is analyzed using the transient data. • A simplified model is used to assess the factors governing annular ice growth. 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. [ABSTRACT FROM AUTHOR]

Spatial compartmentalization of metabolic pathways within membrane-separated organelles is key to the ability of eukaryotic cells to precisely regulate their biochemical functions. Membrane-bound organelles such as mitochondria, endoplasmic reticulum (ER) and lysosomes enable the concentration of metabolic precursors within optimized chemical environments, greatly accelerating the efficiency of both anabolic and catabolic reactions, enabling division of labor and optimal utilization of resources. However, metabolic compartmentalization also poses a challenge to cells because it creates spatial discontinuities that must be bridged for reaction cascades to be connected and completed. To do so, cells employ different methods to coordinate metabolic fluxes occurring in different organelles, such as membrane-localized transporters to facilitate regulated metabolite exchange between mitochondria and lysosomes, non-vesicular transport pathways via physical contact sites connecting the ER with both mitochondria and lysosomes, as well as localized regulatory signaling processes that coordinately regulate the activity of all these organelles. This review covers how cells use membrane transporters, membrane contact sites, and localized signaling pathways to mediate inter-organelle communication and coordinate metabolism. We also describe how disruption of inter-organelle communication is an emerging driver in a multitude of diseases, from cancer to neurodegeneration. Effective communication among organelles is essential to cellular health and function. Identifying the major molecular players involved in mediating metabolic coordination between organelles will further our understanding of cellular metabolism in health and lead us to design better therapeutics against dysregulated metabolism in disease. [ABSTRACT FROM AUTHOR]

• The effect of channel diameter on flow freezing in microchannels is experimentally investigated. • Freezing starts as dendritic ice followed by annular ice growth that causes channel closure. • The closing time is shown to decreases monotonically with a decrease in the channel diameter. • An extremely short closing time of 0.25 s is observed for a 100 μm inner diameter channel. 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. [ABSTRACT FROM AUTHOR]