Droplet evaporation plays a vital role in various fields of natural science and engineering such as: cloud physics, burning liquid-fuels, air/fuel-premixing, (biological) crystal growth, and painting. Moreover, the high heat transfer rates associated with evaporation suggests its use in contexts with a variety of thermal applications, such as spray-cooling or in the electronics industry for cooling of integrated circuits with high heat dissipation rates. Probably the most common application involving droplet handling and evaporation is ink-jet printing. Using piezo or thermally actuated print heads droplet sizes down to a few picoliters are generated. While home and common office applications rely on aqueous inks containing dyes or pigments, versatile printers suitable for polymer substrates use solvent inks with volatile organic liquids. Yet, ink-jet technology has a potential far beyond these applications, for instance in (bio-)chemistry or electronics, where circuit boards can be inexpensively manufactured by printing the electronic components.
Besides these applications, droplet evaporation reveals several intriguing phenomena and is a prime example of a microfluidic multiphysics system, which is noticeably complex due to the interplay of mass and heat transfer, complex geomteries, interface energy effects as contact angle hysteresis, volumetric forces as gravity, and Marangoni flows.
The evaporation of a liquid drop on a solid substrate is a remarkably common phenomenon. Yet, the complexity of the underlying mechanisms has constrained previous studies to spherically symmetric configurations. Here we investigate well-defined, non-spherical evaporating drops of pure liquids and binary mixtures. We deduce a universal scaling law for the evaporation rate valid for any shape and demonstrate that more curved regions lead to preferential localized depositions in particle-laden drops. Furthermore, geometry induces well-defined flow structures within the drop that change according to the driving mechanism. In the case of binary mixtures, geometry dictates the spatial segregation of the more volatile component as it is depleted. Our results suggest that the drop geometry can be exploited to prescribe the particle deposition and evaporative dynamics of pure drops and the mixing characteristics of multicomponent drops, which may be of interest to a wide range of industrial and scientific applications.
Press: Imperial College London News
The evaporation of non-axisymmetric sessile drops is studied by means of experiments and three-dimensional direct numerical simulations (DNS). The emergence of azimuthal currents and pairs of counter-rotating vortices in the liquid bulk flow is reported in drops with non-circular contact area. These phenomena, especially the latter, which is also observed experimentally, are found to play a critical role in the transient flow dynamics and associated heat transfer. Non-circular drops exhibit variable wettability along the pinned contact line sensitive to the choice of system parameters, and inversely dependent on the local contact-line curvature, providing a simple criterion for estimating the approximate contact-angle distribution. The evaporation rate is found to vary in the same order of magnitude as the liquid–gas interfacial area. Furthermore, the more complex case of drops evaporating with a moving contact line (MCL) in the constant contact-angle mode is addressed. Interestingly, the numerical results demonstrate that the average interface temperature remains essentially constant as the drop evaporates in the constant-angle (CA) mode, while this increases in the constant-radius (CR) mode as the drops become thinner. It is therefore concluded that, for increasing substrate heating, the evaporation rate increases more rapidly in the CR mode than in the CA mode. In other words, the higher the temperature the larger the difference between the lifetimes of an evaporating drop in the CA mode with respect to that evaporating in the CR mode.