Efficient coalescence of two spreading drops

液滴聚结是两个可混溶的液滴在彼此靠近之后在它们之间的“质量传递”过程。这可以在自然过程（雨滴和露水凝结）和人工工程过程（薄膜生长、喷墨印刷、喷涂等）中观察到。同时扩散的液滴的受控液滴聚结对于如上所述的许多应用是至关重要的方面。这种应用中的聚结动力学决定了两个铺展液滴之间的有效质量传递，铺展液滴的低效聚结可能导致几种不期望的效果，例如更长的印刷时间、印刷油墨的过度使用以及在喷洒农药或飞机除冰期间对环境有害的液体的浪费和溢出（高达~50%）。为了显著减少这种损失，在本研究计划中，提供了精确的方法来控制基板上的液滴聚结。 给定研究计划的具体目标是提出有效聚结的途径，这将最终消除由于不明力量而发生的不良影响。这将通过研究迄今为止被忽视的内在吸引-排斥力的作用来获得，特别是与聚结动力学有关的。在液滴铺展过程中，可以观察到两个不同的长度尺度（宏观和微观）-毛细管长度尺度和分子尺度。具有非常小的平衡接触角的液滴通过前体膜聚结。通常，前体膜为几纳米，并且在该尺度下，吸引-排斥力可以改变聚结动力学。因此，在液滴聚结期间需要考虑吸引-排斥力。 因此，在这项拟议的研究中，吸引-排斥力（分子和微观现象）对液滴聚结的宏观性质的影响将被详细研究。 预计该程序将提供一种确定的方法来控制基板上的液滴聚结，并且我们还将填补当前文献中的空白，特别是它将提供对几种无法解释的非直观和不常见现象的解释-延迟聚结，聚结前一个液滴追逐另一个液滴，聚结前较长的延迟弛豫时间，以及聚结期间的自增强振荡，这种最佳且精确的聚结方式将成为喷涂、喷墨印刷、农药喷洒和飞机除冰等多个工业过程的范式转变，这将显著减少有毒和对环境有害的液体的使用。

Liquid drop coalescence is a `mass transfer' process between two miscible liquid drops after being brought in the vicinity of each other. This can be observed in both natural (rain drops and condensation of dew) and artificially engineered processes (film growth, inkjet printing, spraying, etc.). Controlled drop coalescence of simultaneously spreading drops is a crucial aspect for a number of applications as mentioned above. Coalescence dynamics in such applications dictates the efficient mass transfer between two spreading drops, and inefficient coalescence of spreading drops may result in several undesirable effects such as longer printing time, overuse of printing ink and wastage and spillage of environmentally hazardous liquids (up to ~50%) during spraying of pesticides or de-icing of planes. In order to significantly reduce such losses, in this research program the precise way to control the drop coalescence on the substrate is provided. The specific aim of the given research program is to propose the pathway for efficient coalescence which will ultimately eliminate undesirable effects occurring due to unaccounted forces. This will be obtained by investigating the role of inherent attractive-repulsive forces that have been neglected so far, particularly pertaining to the coalescence dynamics. During the process of drop spreading, two distinct length scales (macro and microscopic) can be observed - capillary length scale and molecular scale. The drops with a very small equilibrium contact angle coalesce through the precursor film. Generally, the precursor film is of a few nanometers and at this scale the attractive-repulsive forces may change the coalescence dynamics. Therefore, attractive-repulsive forces need to be considered during drop coalescence. Thus, in this proposed research, the impact of attractive-repulsive forces (molecular and microscopic phenomena) on the macroscopic nature of drop coalescence will be studied in detail.  It is expected that this program will provide a defined way to control drop coalescence on the substrate and it we will also fill the gaps in the current literature, particularly it will provide an explanation to several unexplained non-intuitive and uncommon phenomena - delayed coalescence, one drop chasing another drop before coalescence, longer delay-relaxation time before coalescence, and self-augmented oscillation during the coalescence, etc. This optimum and precise way of coalescence will be a paradigm shift in several industrial processes such as spray coating, inkjet printing, spraying of pesticides and de-icing of airplanes etc., which will significantly reduce the use of toxic and environmentally hazardous liquids.