Abstract
Sessile droplet evaporation underpins a wide range of applications from inkjet printing to coating. However, drying times can be variable and contact-line pinning often leads to undesirable effects, such as ring stain formation. Here, we show voltage programmable control of contact angles during evaporation on two pinning-free surfaces. We use an electrowetting-on-dielectric approach and Slippery Liquid-Infused Porous (SLIP) and Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) surfaces to achieve a constant contact angle mode of evaporation. We report evaporation sequences and droplet lifetimes across a broad range of contact angles from 105°–67°. The values of the contact angles during evaporation are consistent with expectations from electrowetting and the Young-Lippman equation. The droplet contact areas reduce linearly in time, and this provides estimates of diffusion coefficients close to the expected literature value. We further find that the total time of evaporation over the broad contact angle range studied is only weakly dependent on the value of the contact angle. We conclude that on these types of slippery surfaces, droplet lifetimes can be predicted and controlled by the droplet’s volume and physical properties (density, diffusion coefficient, and vapor concentration difference to the vapor phase) largely independent of the precise value of contact angle. These results are relevant to applications, such as printing, spraying, coating, and other processes, where controlling droplet evaporation and drying is important.
Highlights
The evaporation of sessile droplets of liquids from solids occurs in many applications including heat exchange,1 particle deposition,2 and inkjet printing.3 Due to its importance to a wide range of physical processes, the literature is extensive
If the evaporating droplet is a suspension, it can cause nonuniform particle deposition, similar to the coffee-ring stain effect.6. This can cause problems in a broad range of applications from nonuniform delivery of the active components in aerosols used in pesticides to nonuniform fluorescence in spotted microarrays.2,7−9 One way to prevent ring-stain patterns is to remove contact line pinning so that the contact line is completely mobile during evaporation, but this is the exception on solid surfaces unless active means, such surface acoustic wave10 or electrowetting-induced agitation of the liquid, are used
We observed that after a brief initial period, the contact angle remained approximately constant during the evaporation for most of the evaporation period on both types of slippery surfaces (Figure 2)
Summary
The evaporation of sessile droplets of liquids from solids occurs in many applications including heat exchange, particle deposition, and inkjet printing. Due to its importance to a wide range of physical processes, the literature is extensive (see, e.g., Erbil, Cazabat and Gueń a,5 and Larson). An alternative approach to removing contact line-pinning, which avoids the risk of an impalement transition, is the use of a Slippery Liquid-Infused Porous Surface (SLIPS), and this has been shown to support a constant contact angle type mode of evaporation.. Electrowetting is an important tool that can manipulate and control droplets, e.g., in microfluidics,− liquid lenses, and optofluidics, and can be used with SLIP surfaces (see e.g., ref 30−32) In this type of electrowetting, the solid−liquid contact area of a sessile droplet acts as one electrode in a capacitive structure allowing the contact angle to be reduced by the application of a voltage. The total droplet lifetime during constant contact angle evaporation of a spherical cap shaped sessile droplet depends on the value of the contact angle and the initial contact radius (or volume) and a parameter, λ, combining the diffusion coefficient, density of liquid, and the vapor concentration difference
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