McElligott Adam, Guerra André, Wood Michael J, Rey Alejandro D, Kietzig Anne-Marie, Servio Phillip
Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 0C5, Canada.
Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 0C5, Canada.
J Colloid Interface Sci. 2022 Aug;619:84-95. doi: 10.1016/j.jcis.2022.03.082. Epub 2022 Mar 28.
Understanding the crystallization of atmospheric water can require levitation techniques to avoid the influence of container walls. Recently, an acoustic levitation device called the TinyLev was designed, which can levitate multiple droplets at room temperature. Proximal crystallization may affect droplet phase change and morphological characteristics.
In this study, acoustically levitated pure water droplets were frozen individually and in pairs or triplets using a TinyLev device. Nucleation, bulk crystal growth, and melting were observed using digital and infrared cameras concurrently.
Initially, the acoustic field forced the droplets into an oblate spheroid shape, though the counteracting force of the cooling stream caused them to circularize. Droplet geometry was thus the net result of streaming forces and surface tension at the acoustic boundary layer/air-liquid interface. Nucleation was determined to be neither homogeneous nor heterogeneous but secondary, and thus dependent on the cooling rate and not on the degree of supercooling. It was likely initiated by aerosolized ice particles from the air or from droplets that had already nucleated and broken up. The latter secondary ice production process resulted in multi-drop systems with statistically identical nucleation times. Notably, this meant that the presence of interfacial rupture at an adjacent droplet could influence the crystallization behaviour of another. After the formation of an initial ice shell around the individual droplets, dendritic protrusions grew from the droplet surface, likely seeded by the same ice particles that caused nucleation, but at a quasi-liquid layer. When freezing was complete, it was determined that the frozen core had undergone a volumetric expansion of 30.75%, compared to 9% for pure, sessile water expansion. This significantly greater expansion may have resulted from entrained air bubbles at the inner solid-liquid interface and oscillations at the moving phase boundary caused by changes in local acoustic forces. Soon after melting began, acoustic streaming, the buoyancy of the remaining ice, and convective currents caused by both an inner thermal gradient and thermocapillary effects along the air-liquid interface, all contributed to the droplet spinning about the horizontal axis.
要理解大气水的结晶过程可能需要悬浮技术，以避免容器壁的影响。最近，设计了一种名为TinyLev的声悬浮装置，它可以在室温下悬浮多个液滴。邻近结晶可能会影响液滴的相变和形态特征。
在本研究中，使用TinyLev装置将声悬浮的纯水液滴单独以及成对或三个一组地进行冷冻。同时使用数字相机和红外相机观察成核、整体晶体生长和熔化过程。
最初，声场迫使液滴呈扁球体形状，不过冷却流的反作用力使其变为圆形。因此，液滴的几何形状是声边界层/气液界面处的流动作用力和表面张力的综合结果。成核过程被确定既不是均相成核也不是异相成核，而是二次成核，因此取决于冷却速率而非过冷度。它可能是由空气中的雾化冰颗粒或已经成核并破碎的液滴引发的。后一种二次冰生成过程导致多液滴系统具有统计学上相同的成核时间。值得注意的是，这意味着相邻液滴处界面破裂的存在可能会影响另一个液滴的结晶行为。在单个液滴周围形成初始冰壳后，树枝状突起从液滴表面生长，可能是由导致成核的相同冰颗粒作为晶种，但在准液层中。冷冻完成后，确定冷冻核心的体积膨胀了30.75%，相比之下，纯静态水的膨胀率为9%。这种显著更大的膨胀可能是由于内部固液界面处夹带的气泡以及局部声力变化在移动相边界处引起的振荡所致。熔化开始后不久，声流、剩余冰的浮力以及由内部热梯度和沿气液界面的热毛细效应引起的对流，都导致液滴绕水平轴旋转。
