Characterization of spontaneous transformation-based droplet formation during microchannel emulsification
Characterization of spontaneous transformation-based droplet formation during microchannel emulsification
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DOI：
10.1021/jp0259871
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发表时间：
2002-09-12
影响因子：
3.3
通讯作者：
Seki, M
中科院分区：
文献类型：
--
作者：
Sugiura, S;Nakajima, M;Seki, M
最近，我们提出了微通道乳化技术，这是一种从微通道阵列制备单分散乳液的新方法。MC乳化的液滴形成机理是一种独特的机理，即扭曲的分散相在界面张力作用下自发转化为球形液滴。本研究的目的是描述MC乳化过程中的流动特性。用不同尺寸和形状的MC考察了分散相在不同流速下的乳化行为，发现了一个临界流速，在这个临界流速下，流动特性发生了很大的变化。形成的液滴直径在临界速度以下几乎不变，形成了单分散的乳状液。液滴直径在临界速度以上急剧增大，形成多分散乳状液。临界速度与MC尺寸无关。无量纲数分析表明，MC乳化液流动特性发生变化的临界点取决于毛细管数(Ca)，即粘性力与界面张力之比。在临界Ca以下，液滴形成的驱动力--界面张力占主导地位，MC乳化过程中的流动以自发转变为主。在临界钙以上，粘性力占主导地位，流动类似于层流流动。我们使用不同粘度的分散相和连续相、不同尺寸的MC和不同的界面张力对这一观点进行了实验验证。结果表明，MC乳化过程具有相似规律，流动的转变由Ca决定。本研究所获得的信息包含了MC乳化过程中自发相变流动的基本物理基础，对实际应用具有一定的参考价值。
Recently, we proposed a microchannel (MC) emulsification technique, which is a novel method for making a monodispersed emulsion from a microfabricated channel array. The droplet formation mechanism for MC emulsification is a unique one, in which the distorted dispersed phase is spontaneously transformed into spherical droplets by interfacial tension. The objective of this study was to characterize the flow in MC emulsification. We investigated the emulsification behavior at different flow velocities of the dispersed phase using MCs with different sizes and analogous shapes and found a critical flow velocity over which the character of flow changed drastically. The formed droplet diameters were almost constant below the critical velocity, and monodispersed emulsions were formed. The droplet diameters increased drastically above the critical velocity, and polydispersed emulsions were formed. The critical velocities were independent of MC size. Analysis using a dimensionless number revealed that the critical point, at which the character of the flow of MC emulsification changes, is determined by the Capillary number (Ca), which is the ratio of viscous force to interfacial tension force. Below the critical Ca, the interfacial tension, which is the driving force for droplet formation, is dominant, and the flow in MC emulsification was based on spontaneous transformation. Above the critical Ca, viscous force is dominant, and the flow is similar to laminar flow. We experimentally confirmed this idea using the dispersed and continuous phases with different viscosities, MCs with different sizes, and different interfacial tensions. These results suggested a law of similarity in MC emulsification, and the transition of the flow was determined by Ca. The information obtained in this study includes the essential physics underlying spontaneous transformation flow in MC emulsification and is useful for practical applications.