Abstract:We demonstrate bendotaxis, a novel mechanism for droplet self-transport at small scales. A combination of bending and capillarity in a thin channel causes a pressure gradient that, in turn, results in the spontaneous movement of a liquid droplet. Surprisingly, the direction of this motion is always the same, regardless of the wettability of the channel. We use a combination of experiments at a macroscopic scale and a simple mathematical model to study this motion, focussing in particular on the time scale asso… Show more
“…We, therefore, call this elastocapillary mechanism, the essence of which is the deformable geometry and boundaries. Similar concepts have been recently explored for the control of passive motions of micron-sized droplets, including durotaxis 46 , tensotaxis 47 , and bendotaxis 48 . Fig.…”
Although layered van der Waals (vdW) materials involve vast interface areas that are often subject to contamination, vdW interactions between layers may squeeze interfacial contaminants into nanopockets. More intriguingly, those nanopockets could spontaneously coalesce into larger ones, which are easier to be squeezed out the atomic channels. Such unusual phenomena have been thought of as an Ostwald ripening process that is driven by the capillarity of the confined liquid. The underlying mechanism, however, is unclear as the crucial role played by the sheet’s elasticity has not been previously appreciated. Here, we demonstrate the coalescence of separated nanopockets and propose a cleaning mechanism in which both elastic and capillary forces are at play. We elucidate this mechanism in terms of control of the nanopocket morphology and the coalescence of nanopockets via a mechanical stretch. Besides, we demonstrate that bilayer graphene interfaces excel in self-renewal phenomena.
“…We, therefore, call this elastocapillary mechanism, the essence of which is the deformable geometry and boundaries. Similar concepts have been recently explored for the control of passive motions of micron-sized droplets, including durotaxis 46 , tensotaxis 47 , and bendotaxis 48 . Fig.…”
Although layered van der Waals (vdW) materials involve vast interface areas that are often subject to contamination, vdW interactions between layers may squeeze interfacial contaminants into nanopockets. More intriguingly, those nanopockets could spontaneously coalesce into larger ones, which are easier to be squeezed out the atomic channels. Such unusual phenomena have been thought of as an Ostwald ripening process that is driven by the capillarity of the confined liquid. The underlying mechanism, however, is unclear as the crucial role played by the sheet’s elasticity has not been previously appreciated. Here, we demonstrate the coalescence of separated nanopockets and propose a cleaning mechanism in which both elastic and capillary forces are at play. We elucidate this mechanism in terms of control of the nanopocket morphology and the coalescence of nanopockets via a mechanical stretch. Besides, we demonstrate that bilayer graphene interfaces excel in self-renewal phenomena.
“…Similarly to the no tilting case, in writing down the torque balance (3.14) we have neglected the rotational inertia of the plate, Iα, where I = m 2 /3 is the moment of inertia for a plate of uniform density, width 2 and mass per unit length m. To understand when this neglect of rotational inertia is appropriate, we note that a droplet centred on the pivot will induce a torque on the plate because the pressure at its menisci becomes unequal immediately following a rotation α. The magnitude of this capillary torque, τ cap ∝ ∆ p x 2 0 where the pressure difference ∆ p ∝γαx 0 /h 2 0 (Bradley et al, 2019); we therefore have that τ cap ∝γαx 3 0 /h 2 0 dominates the rotational inertia for times t mh 2 0 2 /(γx 3 0 ) 1/2 . This holds in particular for times on the capillary time scale t * provided that W /γ m 2γ 2 4 /(µ…”
Section: Hinged Plate: Only Tiltingmentioning
confidence: 81%
“…For wetting droplets (as considered here), such a tapering-induced pressure difference will tend to drive the droplet towards the narrower end, as was first observed by Hauksbee (1710). This confinement-induced migration has been studied quantitatively recently via simulations (Concus & Finn, 1998;Ruiz-Gutiérrez et al, 2018;Ruiz-Gutiérrez & Ledesma-Aguilar, 2020) and experimentally (Renvoisé et al, 2009;Reyssat, 2014;Bradley et al, 2019). (For non-wetting drops, motion occurs towards the wider end in 2D, while in 3D droplet equilibria are possible, provided that the surface is sufficiently non-wetting, dependent on the wedge angle, as studied by Concus & Finn, 1998;Concus et al, 2001;Ruiz-Gutiérrez et al, 2018, for example.…”
Section: Detachment Of a Loaded Plate In Two Dimensionsmentioning
We study the dynamics of detachment in 2D capillary adhesion by considering a plate that is initially attached to a flat, rigid substrate via the surface tension of a bridging liquid droplet. In particular, we focus on the effect of allowing the plate to tilt freely during its subsequent motion. A linear stability analysis shows that small perturbations from equilibrium decouple into two modes: one in which the plate separates from the substrate, remaining parallel, and another in which it tilts, simultaneously causing the bridging droplet to migrate. If the initial tilt perturbation is of a similar magnitude to (or bigger than) the separation perturbation, then the presence of this second tilting mode can significantly alter the dynamics. Indeed, this tilting mechanism changes the ultimate fate of the plate: depending on the size of the plate and the initial perturbation, the plate may anomalously detach. We discuss this observation in relation to previous experiments on a 3D system that showed a qualitatively similar anomalous detachment.
“…Directional transport of droplets above a substrate can for instance be reached through a Leidenfrost dynamics above a textured solid (Lagubeau et al 2011) or a Marangoni dynamics of a drop above a liquid film with a temperature gradient (Davanlou & Kumar 2015). Gradients of substrate stiffness (Style et al 2013) or bending rigidity (Bradley et al 2019) can also lead to the transport of a droplet. Our work gives a first minimal description of how gravitational settling dynamics of droplets can be affected by a thin compliant layer, but there are many natural extensions such as how nonlinear elastic effects, gradients in substrate properties and adhesive contact affect the flow.…”
Gravitational settling of a droplet in air onto a soft substrate is a ubiquitous event relevant to many natural processes and engineering applications. We study this phenomenon by developing a three-phase lubrication model of droplet settling onto a solid substrate coated by a thin soft layer represented by a viscous film, an elastic compressible layer and an elastic sheet supported by a viscous film. By combining scaling analysis, analytical methods and numerical simulations we elucidate how the resulting droplet dynamics is affected by the nature of the soft layer. We show that these soft layers can significantly affect the droplet shape during gravitational settling. When there is a linear response of the deformations of the soft layer, the air layer takes longer to drain as compared with the case of a droplet settling onto a rigid substrate. Our results provide new insight into the coupled interactions between droplets and solids coated by a thin film of a soft material.
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