Fig. 2: Illustration of a droplet on a liquid-infused surface with topographical gradient.
From: Bidirectional motion of droplets on gradient liquid infused surfaces
a The droplet and lubricant meniscus are represented by the turquoise and orange discs, respectively. The greater solid fraction (fs) area is indicated by the darker colour. R and r are the droplet base radius and meniscus width, respectively. Γout and Γin are the surface tension forces per unit length that act on the outer and inner contact lines, with ϕ the angle in the azimuthal direction, and dl and dL the infinitesimal length of the inner and the outer contact lines respectively. b, c Depending on the sign of the spreading parameter S, the lubricant may encapsulate the droplet. d Magnification of the meniscus area (side-view). The dashed line at the droplet–air interface indicates the possibility of lubricant encapsulation, while the grey rectangles represent the surface topography, infused with lubricant (orange). The red arrows indicate the relevant interfacial tensions γ(s, o)α of the composite solid-lubricant surface with fluid phase α. e–h Possible wetting states depending on the lubricant contact angle on the solid surface in the droplet (θow) and air (θoa) environments.
