Feeling through a film

Touch a liquid surface with a sharp needle and pull it gently – the force you exert is essentially determined by the surface tension of the liquid. This classical capillary phenomenon was revisited in a recent paper, published in Physical Review Letters, when a thin elastic film floats on the liquid surface. The presence of a film increases the force that is needed to poke the interface, as might be expected. Surprisingly, however, it is not the 3POKING newmechanical properties of the film that stiffen the interface, but rather the interplay between surface tension, hydrostatic pressure and the finite size of the sheet. This universal, material-independent response, is attributed to a novel geometrical concept that the authors call “asymptotic isometry”: the film is so thin that it easily forms very fine wrinkles and in so doing eliminates any strain within itself. How then does stiffening of the interface occur? The role of the film becomes solely to transmit the effect of poking to a larger region of the liquid than would be possible with a regular meniscus. Controlled experiments that confirmed this scenario employed ultra-thin polymer films, but the same geometrical principle is expected to govern the mechanics of poking in various other materials, including the thin layer that forms on warm milk or custard.

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Drop deforms a thin solid

Imagine a liquid drop resting on the surface of a solid material. The classical picture, due to Young and Laplace, assumes that the drop rests on the substrate without deforming it. However, if the solid is sufficiently thin, the capillary forces affect its shape dramatically, yielding a pattern of wrinkles that emanate from the liquid-solid contact line (see attached figure). Furthermore, the contact angle deviates from the classical Young’s law, which only accounts for surface energies and ignores the solid’s elasticity. In a paper, published in Physical Review Letters (R.D. Schroll et al. , 2013) we analyzed this effect and found that it is attributed to the high bendability of thin solid objects. This insight  suggests new regimes of “elasto-capillary” phenomena, governed by surface energy, elasticity and geometry. The characterization of the wrinkle pattern enabled by this theory opens the way to new quantitative techniques for the metrology of nano-metrically thin solid sheets.

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Figure: Image (interferometry) of an ultrathin film near a solid-liquid-vapor contact line.

 

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A sheet on a drop presents: wrinkle-to-crumple transition

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Smooth wrinkles and sharply crumpled regions are familiar motifs in biological or synthetic sheets, such as rapidly growing plant leaves and crushed foils. Nevertheless, the generic route whereby a featureless sheet develops a complex shape remains elusive. Think for instance … Continue reading

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The curtain problem

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The question underlying pattern formation theory is how a complex shape emerges under featureless forces and constraints. The basic paradigm here relies on a fundamental principle in modern condensed matter theory: A spontaneous breaking of continuos symmetry. In a previous … Continue reading

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Smooth cascades

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How does one make a rippled sheet terminate at a straight edge? If the sheet is sufficiently thin, such as a piece of paper or fabric, then the obvious solution of stretching it out flat will induce large stresses near … Continue reading

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Understanding Wrinkling

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Wrinkling patterns are ubiquitous in elastic sheets of various types: plastic wraps, metallic foils, human skins, or plant leaves. Yet, the strong dependence of these patterns on physical parameters, such as the thickness of the sheet and the stretching forces, … Continue reading

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Elastic building blocks

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The cusp topography of a crumpled paper appears markedly different from a smooth wrinkled skin. Nevertheless, a work that was published recently in Phys.Rev.Lett. suggests that both morphologies may simply reflect different coexistence forms between a few common “elastic building blocks”. … Continue reading

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