Bursting of Volatile Sessile Drops Induced by Ambient Moisture

Abstract

When a pure drop evaporates on a wettable substrate, the evaporation typically proceeds in an orderly fashion and contact line breakups are not expected. We report a surprising observation on an initially pure solvent drop (e.g. of isopropanol), which bursts into thousands of tiny daughter droplets while evaporating on a wettable solid substrate. The bursting mechanism is uncovered by integrating experiments, modeling and simulations. Experimentally, we identify the crucial yet largely ignored role of ambient humidity in inducing the bursting phenomenon. The bursting only occurs at a high humidity of water vapor and on a substrate that is partially wetting to water. To elucidate the moisture-induced bursting mechanism, we build a thin-film flow model that accounts for the phase change processes of the binary mixture of solvent and water. The model permits simultaneous evaporation of the solvent drop and absorption of the water vapor. The numerical simulations based on this model captures the bursting phenomenon and indicates a two-step process leading to bursting: In the advancing phase, the initially pure solvent drop absorbs water vapor from the surrounding air, producing a binary solvent-water drop; The solutal Marangoni stress creates a water-rich ridge at the advancing contact line of the spreading drop. In the receding phase, the underlying substrate that is partially wetting to water forces the water-rich ridge to break up from the solvent-rich main drop; Successive breakups drive the receding drop to burst into tiny droplets. The integrated experimental and numerical investigations confirm the crucial role of relative humidity and substrate wettability. Our mechanistic understanding is relevant to a wide variety of evaporation processes involving a volatile solvent evaporating in the ambient air, as solid substrates are likely hydrophobic after air exposure.In Appendix A, electrostatic actuation techniques are discussed for both thin-film (wetting) drops and superhydrophobic (non-wetting) drops. For thin-film drops, the electrostatic technique may be used to control the deposition and spreading of a volatile liquid. For superhydrophobic drops, the electrostatic technique offers non-contact actuation of aqueous solutions while minimizing the solid-liquid contact. In Appendix B, analytical solutions are presented for heat conduction problems with either isothermal or uniform heat flux that is localized at a boundary region. The isothermal flux solution is useful for estimating the temperature reduction due to evaporative cooling and is related to the well-known Weber disk solution for evaporative mass flux, with temperature (or vapor density) gradient driving heat (or mass) flux. The uniform flux solution is useful for studying the effect of hotspot size on conductive heat spreading.