Recently, there has been much interest in using lubricated flat and nano-/micro-structured surfaces to achieve extreme liquid-repellency: any foreign droplet immiscible with the underlying lubricant layer was shown to slide off at a small tilt angle $<$ 5$^{\circ}$. This behavior was hypothesized to arise from a thin lubricant overlayer film sandwiched between the droplet and solid substrate, but this has not been observed experimentally. Here, using confocal optical interferometry, we are able to visualize the intercalated film under both static and dynamic conditions. We further demonstrate that the lubricant flow entrained by droplet motion can transform a partially dewetted film into a continuous layer, by generating a sufficient hydrodynamic force to lift the droplet over the solid substrate. The droplet is therefore oleoplaning, akin to tires hydroplaning on a wet road, with minimal dissipative force (down to 0.1 $\mu$N for 1 $\mu$l droplet when measured using a cantilever force sensor) and no contact line pinning. The techniques and insights presented in this study will inform future work on the fundamentals of wetting for lubricated surfaces and enable their rational design
Approaches for regulated fluid secretion, which typically rely on fluid encapsulation and release from a shelled compartment, do not usually allow for a fine, continuous modulation of secretion, and can be difficult to adapt for monitoring or functionintegration purposes. 1-5 Here, we report self-regulated, self-reporting secretion systems consisting of liquid-storage compartments in a supramolecular polymer-gel matrix with a thin liquid layer on top, and demonstrate that dynamic liquid exchange between the compartments, matrix and surface layer allows for repeated, responsive self-lubrication of the surface layer and for cooperative healing of the matrix. Depletion of the surface liquid or local material damage induces self-regulated secretion of the stored liquid via a dynamic feedback between polymer crosslinking, droplet shrinkage and liquid transport that can be read out through changes in the system's optical transparency. We envision diverse applications in fluid delivery, wetting and adhesion control, and material self-repair.Nearly every form of living tissue autonomously packages, transports, and secretes fluids, mediating defense, adhesion, wound healing, temperature -often several of these at once -through tightly self-regulated release systems. [6][7][8][9] Fundamental to these systems, fluid storage is itself an active, finely regulated balance. Storage droplets or vesicles continuously adjust their size, shape and contents through ongoing exchange with the surroundings, creating intrinsically responsive control mechanisms that tie secretion to a wide range of chemical and physical stimuli and feedback signals. [10][11][12][13] At the same time, collective changes in the stores are reported to the organism, alerting it that it needs to drink or eat to replenish the limited supply. Many synthetic approaches have been developed to enable triggered release from microcapsules, hydrogels, nanoparticles, vesicles, micelles, mesoporous carriers and other containers. [1][2][3][4][5][14][15][16][17] While these systems can secrete fluid in response to various stimuli, it remains a challenge to design a synthetic approach that displays finely tuned, continuous self-adjustment, integrated functionalities, and continuous liquid supply monitoring.2 Figure 1. Schematic of the self-regulated, liquid secretion system. Secretion liquid is stored as shell-less droplets inside a gel matrix composed of dynamic polymers, with ongoing liquid exchange between droplet and gel phases. If S = γ ga -(γ la + γ gl ) > 0, the matrix surface will be coated with a thin liquid overlayer. When this layer is removed, the disjoining pressure will trigger secretion of the stored liquid to restore the original film thickness, while the supramolecular gel matrix reconfigures through reversible bond disassembly and reassembly to release any buildup of mechanical stress due to shrinking droplets. With successive removal/restoring cycles, the liquid droplets will continuously shrink and the gel will become progressively transparent.Inspire...
Lubricated surfaces have shown promise in numerous applications where impinging foreign droplets must be removed easily; however, before they can be widely adopted, the problem of lubricant depletion, which eventually leads to decreased performance, must be solved. Despite recent progress, a quantitative mechanistic explanation for lubricant depletion is still lacking. Here, we first explained the shape of a droplet on a lubricated surface by balancing the Laplace pressures across interfaces. We then showed that the lubricant film thicknesses beneath, behind, and wrapping around a moving droplet change dynamically with droplet's speed-analogous to the classical Landau-Levich-Derjaguin problem. The interconnected lubricant dynamics results in the growth of the wetting ridge around the droplet, which is the dominant source of lubricant depletion. We then developed an analytic expression for the maximum amount of lubricant that can be depleted by a single droplet. Counter-intuitively, faster moving droplets subjected to higher driving forces deplete less lubricant than their slower moving counterparts. The insights developed in this work will inform future work and the design of longer-lasting lubricated surfaces. * These two authors contributed equally † jaiz@seas.harvard.edu arXiv:1807.03934v2 [physics.flu-dyn]
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