Elastocapillary sequential fluid capture in hummingbird-inspired grooved sheets
Emmanuel Siéfert, Benoît Scheid, Fabian Brau, Jean Cappello
Abstract
Passive and effective fluid capture and transport at small scale is crucial for industrial and medical applications, especially for the realisation of point-of-care tests. Performing these tests involves several steps, including capturing biological fluid, aliquoting, reacting with reagents, and reading the results. Ideally, these tests must be fast and offer a large surface-to-volume ratio to achieve rapid and precise diagnostics with a reduced amount of fluid. Such constraints are often contradictory as a high surface-to-volume ratio implies a high hydraulic resistance and hence a decrease in the flow rate. Inspired by the feeding mechanism of hummingbirds, we propose a frugal fluid capture device that takes advantage of elastocapillary deformations to enable concomitant fast liquid transport, aliquoting, and high confinement in the deformed state. The hierarchical design of the device - that consists in vertical grooves stacked on an elastic sheet - enables a two-step sequential fluid capture. Each unit groove mimics the hummingbird's tongue and closes due to capillary forces when a wetting liquid penetrates, yielding the closure of the whole device in a tubular shape, in the core of which additional liquid is captured. Combining elasticity, capillarity, and viscous flow, we rationalise the fluid-structure interaction at play both when liquid is scarce and abundant. By functionalising the surface of the grooves, such a passive device can concomitantly achieve all the steps of point-of-care tests, opening the way for the design of optimal devices for fluid capture and transport in microfluidics.