Department of Chemical and Biomolecular Engineering North Carolina State University, 911 Partners Way, Raleigh, NC 27695, USA.
Lab Chip. 2012 Oct 21;12(20):3961-7. doi: 10.1039/c2lc40492c.
This manuscript describes an experimental study of the production of micro-scale droplets of the room-temperature liquid alloy eutectic gallium indium (EGaIn) formed using a microfluidic flow-focusing device. The EGaIn surface oxidizes readily to form a passivating oxide "skin" that imparts some mechanical stability to the resulting microspheres, but does not appear to affect the dynamics of droplet formation. EGaIn has an interfacial tension nearly an order of magnitude larger than typical water-in-oil systems that are used to study droplet production in microfluidic flow-focusing devices. The size of the microdroplets increase as the ratio of the flow rates of the dispersed and continuous-phase increase for both EGaIn-in-glycerol and water-in-oil systems; however, these fluid pairs form droplets through different dispersing modes at otherwise identical flow conditions (i.e., flow rate ratios and capillary numbers). Consequently, the EGaIn droplets are larger than the water droplets. The difference in dispersing modes and droplet size are attributed to the relatively larger interfacial and inertial forces of the EGaIn system compared to the water-in-oil system. The addition of polyvinyl alcohol (PVA), which is known to bind to oxide surfaces, to the continuous phase yields stable, monodisperse emulsions of liquid metal. These emulsions can be destabilized on demand by changing the solution pH, allowing the liquid metal to be recovered. The ability of the PVA to bind to the liquid metal also influences droplet production by changing the shape of the liquid as it approaches the orifice of the flow focusing device, which results in droplets with smaller diameters relative to those formed without PVA.
本文描述了使用微流控流聚焦装置生产室温液态共晶镓铟（EGaIn）微尺度液滴的实验研究。EGaIn 表面容易氧化形成钝化氧化“皮”，赋予所得微球一定的机械稳定性，但似乎不会影响液滴形成的动力学。EGaIn 的界面张力比用于研究微流控流聚焦装置中液滴生成的典型油包水体系大一个数量级。对于 EGaIn-甘油和水包油体系，随着分散相和连续相流速比的增加，微液滴的尺寸增大；然而，这两种流体对在其他相同流动条件下（即流速比和毛细管数）通过不同的分散模式形成液滴。因此，EGaIn 液滴比水液滴大。分散模式和液滴尺寸的差异归因于 EGaIn 体系相对于水包油体系的界面和惯性力较大。向连续相中添加聚乙烯醇（PVA），已知其与氧化表面结合，可得到稳定的单相液态金属乳液。通过改变溶液 pH 值可以按需使这些乳液失稳，从而使液态金属得以回收。PVA 与液态金属结合的能力也通过改变液体在接近流聚焦装置孔口时的形状来影响液滴的生成，从而导致液滴的直径相对于没有 PVA 时形成的液滴的直径更小。
