Elastocapillary Effects Research Articles

Bernoulli catenary and elasto-capillary effect in partially wet fibrous materials

When a wetting liquid wicks into a fibrous material, it causes the material to deform. In this paper we discuss the elasto-capillary effect that leads to spontaneous internal stresses in the materials. The elasto-capillary effect produced by menisci in pores can be identified through a specific stress distribution in the fibrous matrix. We show that the classical Bernoulli problem of a freely hanging fabric can be used for the analysis of gravity-induced stresses in textile materials. These stresses change due to elasto-capillary effect. Wicking of a wetting liquid into a freely suspended fibrous material is shown to be an instructive nontrivial experiment illustrating interesting distinguishable stress distributions in the fibrous matrix.

Direct Measurement of the Surface Tension of a Soft Elastic Hydrogel: Exploration of Elastocapillary Instability in Adhesion

An adhesively stressed thin film of a soft hydrogel confined between two rigid flat substrates autoroughens with its dominant wavelength (λ) exhibiting pronounced dependence on the film thickness (H). A linear stability analysis confirmed that this long wavelength instability (λ ~ 7H) is due to an elastocapillary effect, the implementation of which required direct measurements of the surface tension and the elasticity of the gel. The surface tension of the gel was estimated from the fundamental spherical harmonic of a hemispherical cap of the gel that was excited by an external noise. The shear modulus (μ) of the gel was determined from its resonant shear mode in a confined geometry. During the course of this study, it was found that a high density steel ball submerges itself inside the gel by balancing its excess weight with the accumulated strain induced elastic force that allows another estimation of its elastic modulus. The large ratio (1.8 mm) of the surface tension to its elasticity ascertains the role of elastocapillarity in the adhesion-induced pattern formation with such gels. Experimental results are in accord with a linear stability analysis that predicts that the rescaled wavelength λ(μH/γ)(0.27) is linear with H, which also modifies the conventional stress to pull a flat rigid object out of a very soft film by a multiplicative factor: (γ/μH)(1/4). The analysis also suggests some new results related to the role of the finite dilation of a material in interfacial pattern formation that may have nontrivial consequences in the adhesive delamination of very thin and/or soft elastic films via self-generated cracks.

Wetting of flexible fibre arrays

Fibrous media are functional and versatile materials, as demonstrated by their ubiquity both in natural systems such as feathers and adhesive pads and in engineered systems from nanotextured surfaces to textile products, where they offer benefits in filtration, insulation, wetting and colouring. The elasticity and high aspect ratios of the fibres allow deformation under capillary forces, which cause mechanical damage, matting self-assembly or colour changes, with many industrial and ecological consequences. Attempts to understand these systems have mostly focused on the wetting of rigid fibres or on elastocapillary effects in planar geometries and on a fibre brush withdrawn from an infinite bath. Here we consider the frequently encountered case of a liquid drop deposited on a flexible fibre array and show that flexibility, fibre geometry and drop volume are the crucial parameters that are necessary to understand the various observations referred to above. We identify the conditions required for a drop to remain compact with minimal spreading or to cause a pair of elastic fibres to coalesce. We find that there is a critical volume of liquid, and, hence, a critical drop size, above which this coalescence does not occur. We also identify a drop size that maximizes liquid capture. For both wetting and deformation of the substrates, we present rules that are deduced from the geometric and material properties of the fibres and the volume of the drop. These ideas are applicable to a wide range of fibrous materials, as we illustrate with examples for feathers, beetle tarsi, sprays and microfabricated systems.

Elastocapillarity: Stress transfer through fibrous probes in wicking experiments

Current advances in the manufacture of nanoporous and nanofibrous materials with high absorption capacity open up new opportunities for the development of fiber-based probes and sensors. Pore structures of these materials can be designed to provide high suction pressure and fast wicking. During wicking, due to the strong capillary action, liquids exert stresses on the fiber network. In this paper, we discuss the effect of stress transfer in the direction of propagation of the wetting front. As an illustration, we first consider a single capillary and demonstrate the effect of a moving meniscus on stress distribution along capillary walls. We then analyze similar effects in yarns. We consider a yarn that can capture an aerosol droplet as a promising sensing element that could monitor the stresses caused by wetting fronts. We also discuss the elastocapillary effects occurring during upward and downward wicking. The distributions of stresses in these two cases are shown to differ significantly. We discuss how these effects might be exploited for designing fiber-based sensors that can probe very small amounts of liquids.

Floating Objects with Finite Resistance to Bending

We consider the equilibrium flotation of a thin, flexible cylinder at the interface between a liquid and a gas. In particular, we determine the maximum load that such a cylinder can support without sinking. We find that as the length of such a cylinder increases the maximum load at first increases. However, the maximum load reaches a plateau when the length of the cylinder is comparable to the elastocapillary length, which is determined by a balance between the bending of the cylinder and surface tension. We then consider the implications of our analysis for the walking on water of both arthropods and man-made robots. In particular, we show that the legs of water striders are typically slightly shorter than this 'optimal' length, suggesting that elastocapillary effects may act as a selection pressure.

Columbus Regional Hospital: Creating a Learning Environment

The interaction between capillary force and elastic deformation is known as the elastocapillary effect, which plays an important role in various applications as diverse as self-cleaning surfaces, fog collection, and insect and robot locomotion at wet surfaces. A simple numerical handling of the solid–liquid interfacial coupling remains a challenge due to the highly nonlinear deformation and moving contact lines. This paper presents a simple yet versatile lattice/particle-based numerical method, referred to as the L-MDPD method, to model the elastocapillary effect. This method employs a lattice model to simulate solids and the many-body dissipative particle dynamics model to simulate liquids. The coupling is enabled by tuning the pair interaction between the solid lattices and liquid particles to achieve a desired wetting property. We demonstrate several well-known elastocapillary phenomena, including elastocapillary collapses of elastic beams, rod-wrapping around droplets, and capillary origami, which show a qualitative and quantitative agreement with the reported experimental and theoretical works.