Pressure melting and regelation of ice by round wires
Pressure melting and regelation of ice by round wires
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圆线压力融化和重新凝结冰
DOI：
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发表时间：
1973
期刊：
影响因子：
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通讯作者：
R. L. Shreve
中科院分区：
文献类型：
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作者：
L. Drake;R. L. Shreve
长期以来，人们一直用金属丝前部的压力熔化和后部的再结晶来解释金属丝在冰中横向拉伸的运动，这个过程的速度由熔化热通过金属丝和冰的传导速率控制。通过定量处理，这张简单的图片预测了与驱动应力成正比的焊丝速度，驱动应力定义为每单位长度的驱动力除以焊丝周长的一半。然而，实验观察显示了更为复杂的行为。除了最低的驱动应力外，观察到的速度都非线性地增加，并且在约1巴（105 Pa）的应力下急剧地但连续地且可逆地跳跃，其量的范围从尼龙线的六倍到铜线的60倍。在这种转变之上，高导电性的电线（如铜线）的速度低至预测值的八分之一，而导电性差的电线（如尼龙和铬镍合金）的速度与预测值大致相同。在过渡层以下，所有导线的速度都比预期的要小得多。令人惊讶的是，所有的线速度显着降低的气泡在冰中的存在。导线留下的痕迹在过渡区下方由广泛分散的、通常是微小的水气泡组成，但在过渡区上方，在高导电导线的情况下，从大量的水和蒸汽气泡到导电不良导线的情况下的中心扁平水层。对迹线中水的体积分数的测量表明，在转变温度以上，热量从周围的冰流向运动的导线。非线性和低速度低于过渡是由于存在积累的溶质在水层周围的电线，集中向后方，降低冻结温度，因此，热量流向前方的速度。当后部的温度达到三相点时发生转变，三相点固定了那里的压力，因此随着驱动应力的增加，金属丝周围的平均压力增加，因此平均温度降低，导致热量流向金属丝并形成痕迹，从而带走溶解的溶质。由于冻结表面的弗兰克不稳定性，高导电性导线的痕迹是气泡状的，而不是扁平状的，这使得水和蒸汽的手指生长，直到被表面张力夹断。对于导电性差的导线，过渡区以上的非线性主要是由于导线前部的额外熔化以及与迹线形成相关的导线周围压力分布的变化。对于高导电性的导线，非线性和过渡以上的意外缓慢主要是由于有限的冻结速率所需的过冷，这就像溶解的溶质的存在一样，降低了导线后部的冻结温度。当修改，以近似考虑这些影响，简单的定量处理预测线速度，考虑到描述的溶质含量和所需的过冷度的参数的不确定性，是在良好的协议与实验观察。
The motion of wires pulled transversely through ice has long been explained in terms of pressure melting at the front of the wire and regelation behind it, the speed of the process being controlled by the rate of conduction of the heat of fusion through the wire and the ice. Treated quantitatively, this simple picture predicts wire speeds that are directly proportional to driving stress, defined as driving force per unit length divided by half the circumference of the wire. Experimental observations, however, show much more complicated behaviour. The observed speeds increase nonlinearly at all but the lowest driving stresses, and at a stress of about 1 bar (105 Pa) jump sharply, but continuously and reversibly, by an amount that ranges from six-fold for Nylon wires to 60-fold for copper wires. Above this transition the speeds of highly conductive wires, such as copper, are as low as one-eighth of those predicted, though those of poorly conductive wires, such as Nylon and Chromel, are about the same as predicted. Below the transition the speeds of all wires are much less than predicted. Surprisingly, all wire speeds are significantly reduced by the presence of air bubbles in the ice. The wires leave behind a trace that below the transition consists of widely scattered, generally tiny bubbles of water, but above it grades from numerous bubbles of water and of vapour in the case of highly conductive wires to a central tabular layer of water in the case of poorly conductive ones. Measurements of the fractional volume of water in the trace show that above the transition heat flows to the moving wire from the surrounding ice. The nonlinearity and low speed below the transition are due to the presence of accumulated solutes in the water layer around the wire, which concentrate toward the rear, lowering the freezing temperature there and hence the rate of heat flow toward the front. The transition occurs when the temperature at the rear reaches the triple point, which fixes the pressure there, so that with increasing driving stress the mean pressure around the wire increases and hence the mean temperature decreases, causing heat flow to the wire and formation of the trace, which carries away the dissolved solutes. The trace of highly conductive wires is bubbly, rather than tabular, because of the Frank instability of the freezing surface, which permits fingers of water and vapour to grow until pinched off by surface tension. For poorly conductive wires the nonlinearity above the transition is mainly due to the additional melting at the front of the wire and the change in pressure distribution around the wire associated with the formation of the trace. For highly conductive wires the nonlinearity and unexpected slowness above the transition are mainly due to the supercooling required for a finite rate of freezing, which, like the presence of dissolved solutes, lowers the freezing temperature at the rear of the wire. When modified to take approximate account of these effects, the simple quantitative treatment predicts wire speeds that, considering the uncertainties about the parameters describing the solute content and the required supercooling, are in good agreement with the experimental observations.