Abstract
Sessile drop creation in weightlessness is critical for designing scientific instruments for space applications and for manipulating organic or biological liquids, such as whole human blood or DNA drops. It requires perfect control of injection, spreading, and wetting; however, the simple act of creating a drop on a substrate is more complex than it appears. A new macroscopic model is derived to better understand this related behavior. We find that, for a given set of substrate, liquid, and surrounding gas properties, when the ratio of surface free energies to contact line free energy is on the macroscopic scale, the macroscopic contact angle can vary at static equilibrium over a broad volume range. It can increase or decrease against volume depending on the sign of this ratio up to an asymptotic value. Consequently, our model aims to explore configurations that challenge the faithful representativity of the classical Young’s equation and extends the present understanding of wetting.
Highlights
A sessile drop is a liquid drop deposited on a solid substrate and surrounded by a gaseous environment
It enables the depiction of the full picture in the smallest span range of the drop size, as compared with other dimensionless quantities, h, r, or R, which would require at least one more decade
A new macroscopic mechanical model was derived to compute the shape of weightless sessile drops in static equilibrium
Summary
A sessile drop is a liquid drop deposited on a solid substrate and surrounded by a gaseous environment. Sessile drops are ubiquitous all around us, either in natural environment (raindrops on a surface), or in industrial processes in which liquids intervene. The first answer to this question is provided by the classical Young’s equation[1], Eq (1), which was formulated roughly two centuries ago It remains an undisputed reference for the determination of macroscopic contact angles, denoted by θ (cf Fig. 1), according to its famous relationship: σsg À σsl 1⁄4 σlg cosθ (1)
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