Directional pumping of water and oil microdroplets on slippery surface

Abstract

Transporting water and oil microdroplets is important for applications ranging from water harvesting to biomedical analysis but remains a great challenge. This is due to the amplified contact angle hysteresis and insufficient driving force in the micrometer scale, especially for low-surface energy oil droplets. Coalescence of neighboring droplets, which releases vast additional surface energy, was often required, but its relatively uncontrollable nature brings uncertainties to the droplet motion, and the methodology is not applicable to single droplets. Here we introduce a strategy based on slippery surface with immobilized lubricant menisci to directionally transport microdroplets. By simply mounting hydrogel dots on slippery surface, the raised menisci remotely pump microdroplets via capillary force with high efficiency, regardless of droplet size or surface energy. By proof-of-concept experiments, we demonstrate that our method allows for highly efficient water droplet collection and highly sensitive biomedical analyte detection.droplet transport | microdroplet | capillary force | slippery surface | antifouling

Multibioinspired slippery surfaces with wettable bump arrays for droplets pumping

“…The capillary driving force was written as F = γRðcosα − cosβÞ, where γ represented oil/air interfacial tension and R represented the droplet size (19). Generally, with the increase of R, the capillary driving force increased, whereas bigger droplets also indicated larger droplet mass, bringing about lower initial acceleration and lower pumping speed (SI Appendix, Fig.…”

“…1. Namib desert beetles could attract small water droplets from fog by using their structured backs that are composed of hydrophilic bumps and hydrophobic waxy surroundings (17); Nepenthes pitcher plants have slippery liquid-infused porous (SLIP) structured inwalls that enable them to repel liquids and induce loss-free transportation (18); while for emergent aquatic plants, they could take advantage of capillary driving force for directional pumping of objects with different sizes from a relatively long distance (19). By mimicking these remarkable creatures, many well-performed systems and devices have been designed and reported (20)(21)(22)(23).…”

Fingering Instability of a Gravity-Driven Thin Film Flowing Down a Vertical Tube with Wall Slippage

“…Therefore, using the moving frame ξ = z − ct, we can define a new thickness function H(ξ), where c is the traveling wave speed. In the following analysis, H denotes the thickness function related to variable ξ while h represents the thickness function related to variables z and t. The governing evolution equation (34) becomes an ordinary differential equation related to only one variable:…”

“…For the three dimensional problems, the solution can be simply assumed in the form of H(t, ξ, θ) =H(ξ) + H (t, ξ, θ) [17,23], whereH(ξ) is the traveling wave solution (also called base solution) of Equation (37) and H (t, ξ, θ) is the perturbation. Substituting H(t, ξ, θ) into the governing Equation (34) [17,26], expressed as H (t, ξ, y) =Ĥe iqθ+σ(q)t , where q is the wavenumber and σ(q) relates to the growth rate. For every given wavenumber q, substituting H (t, ξ, y) =Ĥe iqθ+σ(q)t into Equation (38), Equation (38) transforms into a standard eigenvalue equation:…”

“…Recently, inspired by the good antiwetting property of nepenthes pitcher plants, specialists designed various slippery liquid-infused porous substrates similar to the microstructures and functions of pitcher plants [31][32][33]. Moreover, liquid-infused surface with directional slippery lengths was fabricated making use of immobilized lubricant menisci [34]. It is necessary to understand the thin film flow on these biomimetic surfaces, as these structures are usually designed for water collection and transportation.…”