Efficient coalescence of two spreading drops

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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.**