Efficient coalescence of two spreading drops

液滴合并是指两个可混溶的液滴被带到彼此附近后的“传质”过程。这在自然(雨滴和露水凝结)和人工工程(胶片生长、喷墨打印、喷涂等)过程中都可以观察到。同时扩散液滴的受控液滴合并是如上所述的许多应用的关键方面。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传质，而铺展液滴的低效聚结可能会导致几种不良影响，如打印时间较长、印刷油墨过度使用以及在喷洒农药或飞机除冰期间浪费和溢出环境有害液体(高达~50%)。为了显著减少此类损失，本研究计划提供了控制衬底上液滴合并的精确方法。*给定研究计划的具体目标是提出有效合并的途径，这将最终消除由于不明原因的力而产生的不良影响。这将通过研究到目前为止被忽略的内在吸引-排斥力的作用来获得，特别是与合并动力学有关的作用。在液滴扩散过程中，可以观察到两种不同的长度尺度(宏观和微观)--毛细长度尺度和分子尺度。平衡接触角很小的液滴通过前驱体薄膜合并。通常情况下，前驱体薄膜是几个纳米的，在这个尺度上，吸引-排斥力可能会改变合并动力学。因此，在液滴合并过程中需要考虑吸引力-排斥力。因此，在这项拟议的研究中，将详细研究吸引-排斥力(分子和微观现象)对液滴合并的宏观性质的影响。预计该程序将提供一种明确的方法来控制衬底上的液滴合并，它还将填补现有文献中的空白，特别是它将对几种无法解释的非直观和罕见的现象--延迟合并、合并前一个液滴追逐另一个液滴、合并前较长的延迟松弛时间、以及合并过程中的自增强振荡--提供解释。这种最佳和精确的结合方式将是几个工业过程中的范式转变，如喷漆、喷墨打印、喷洒杀虫剂和飞机除冰等，这将大大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.**