Enhancement of the Rate of Surface Reactions by the Elastocapillary Effect.

We examined the effect of deformability of a solid substrate on the kinetics of a surface reaction that occurs between chemical species present in it and a liquid dispensed on it. In particular, we have dispensed aqueous solutions of gold and silver salt as sessile drops or as a liquid pool on a cross-linked film of poly(dimethylsiloxane) (PDMS). The PDMS surface contains organosilane (SiH), which reduces the salt, producing metallic nanoparticles at the solid-liquid interface.

Studying kinetics of a surface reaction using elastocapillary effect.

Hypothesis: When a liquid is inserted inside a microfluidic channel, embedded within a soft elastomeric layer, e.g. poly(dimethylsiloxane) (PDMS), the thin wall of the channel deforms, due to change in solid-liquid interfacial energy.

How does a lizard shed its tail?

Hierarchical microstructures help a lizard self-amputate its tail when needed.

Liquid Spreading Induced by In Situ Generation of Metallic Nanoparticles.

Spreading or pinning of a liquid drop on a solid substrate is determined by the surface energy of solid and liquid, topography of substrate surface, and different external forces like electric field, magnetic field, and vibration. Here we present a novel mechanism of depinning, driven by in situ generation of a species following reaction between a constituent of the droplet and one in the substrate. In particular, fluoro-carbon (FC) functionalized agarose and pHEMA gels are used as the substrates; the substrate is soaked with chloroauric acid.

Fingering instability during fracture of a gel block subjected to shear loading.

We report here an alternative kind of fingering instability observed during fracture of an unconfined gel consisting of two cuboids joined by a thin gel disk, and all prepared monolithically. When the blocks are sheared across the joint, fracture ensues with the appearance of fingers at the fracture front. The spacing between the fingers remains independent of the shearing speed, planar shape of the joint, and the shear modulus of gel.

Confinement-Induced Alteration of Morphologies of Oil-Water Emulsions.

Reversible alteration between different emulsion morphologies like core-shell and Janus is conventionally triggered by altering the interfacial energy between different phases. In contrast, here, we show that the morphology of dispersed droplets can be changed also when the emulsion is sufficiently confined between two parallel plates. In particular, we use three immiscible phases: silicone oil, paraffin oil, and aqueous solution of surface-active agents like agarose, sodium dodecylsulfate, dioctyl sodium sulfosuccinate, and cetyl trimethylammonium bromide to generate oil-in-water emulsions consisting of complex morphologies of the dispersed droplets.

The effect of shape on the fracture of a soft elastic gel subjected to shear load.

For brittle solids, the fracture energy is the energy required to create a unit area of new surface through the process of division. For crosslinked materials, it is a function of the intrinsic properties like crosslinking density and bond strength of the crosslinks. Here we show that the energy released due to fracture can depend also on the shape of a joint made of this material.

Generation of Aspherical Optical Lenses via Arrested Spreading and Pinching of a Cross-Linkable Liquid.

Aspherical optical lenses with spatially varying curvature are desired for capturing high quality, aberration free images in numerous optical applications. Conventionally such lenses are prepared by multistep top-down processes which are expensive, time-consuming, and prone to high failure rate. In this context, an alternate method is presented here based on arrested spreading of a sessile drop of a transparent, cross-linkable polymeric liquid on a solid substrate heated to an elevated temperature.

Estimation of solid-liquid interfacial tension using curved surface of a soft solid.

Unlike liquids, for crystalline solids the surface tension is known to be different from the surface energy. However, the same cannot be said conclusively for amorphous materials like soft cross-linked elastomers. To resolve this issue we have introduced here a direct method for measuring solid-liquid interfacial tension by using the curved surface of a solid.

Adhesion-induced instabilities and pattern formation in thin films of elastomers and gels.

A hydrostatically stressed soft elastic film circumvents the imposed constraint by undergoing a morphological instability, the wavelength of which is dictated by the minimization of the surface and the elastic strain energies of the film. While for a single film, the wavelength is entirely dependent on its thickness, a co-operative energy minimization dictates that the wavelength depends on both the elastic moduli and thicknesses of two contacting films. The wavelength can also depend on the material properties of a film if its surface tension has a pronounced effect in comparison to its elasticity.

A co-operative effect of closely spaced intruding objects puncturing into a soft solid.

When two or more closely spaced indenters puncture a soft solid, the puncturing load diminishes from that required when they are far apart. We have developed a method to examine this effect by embedding a planar tracer grid within a block of an elastic gel and by driving closely spaced syringe needles of different inter-tip spacing through this plane. These experiments show that in addition to a primary crack ahead of the needle-tip, there occurs also a periodically appearing secondary radial crack from its side.

Peeling off an adhesive layer with spatially varying topography and shear modulus.

Inspired by recent experiments on hierarchically structured adhesives, we analyze here the effect of spatial variation in surface topography and shear modulus of an elastomeric adhesive on its ability to adhere strongly to a flexible contactor. The undulation of surface and modulus both were assumed to be periodic with periodicity, which is either identical or different for the two parameters; for identical periodicity, the phase lag between the respective undulations is also systematically varied. Calculations show that during continuous lifting of the flexible contactor from complete initial contact, the interfacial crack between the two adherents does not propagate continuously but intermittently, with crack arrest and initiation at the vicinity of minimum thickness and modulus of the layer; the torque required to initiate an arrested crack increases significantly over that required to propagate it on a smooth adhesive surface.

Harvesting energy of interaction between bacteria and bacteriophage in a membrane-less fuel cell.

When a fuel and oxidant flow in laminar contact through a micro-fluidic channel, a sharp interface appears between the two liquids, which eliminate the need of a proton exchange membrane. This principle has been used to generate potential in a membrane-less fuel cell. This study use such a cell to harvest energy of interaction between a bacteria having negative charge on its surface and a bacteriophage with positive and negative charges on its tail and head, respectively.

Disordered nanowrinkle substrates for inducing crystallization over a wide range of concentration of protein and precipitant.

There are large number of proteins, the existence of which are known but not their crystal structure, because of difficulty in finding the exact condition for their crystallization. Heterogeneous nucleation on disordered porous substrates with small yet large distribution of pores is considered a panacea for this problem, but a universal nucleant suitable for crystallizing large variety of proteins does not really exist. To this end, we report here a nanowrinkled substrate which displays remarkable ability and control over protein crystallization.

Surface-tension-induced flattening of a nearly plane elastic solid.

We report direct measurement of surface deformation in soft solids due to their surface tension. Gel replicas of poly(dimethysiloxane) masters with rippled surfaces are found to have amplitudes that decrease with decreasing gel modulus. Surface undulations of a thin elastomeric film are attenuated when it is oxidized by brief exposure to oxygen plasma.

Control of adhesion via internally pressurized subsurface microchannels.

While pressure sensitive adhesives in general consist of a layer of viscoelastic glue sandwiched between two adherents, we explore here the design of an adhesive embedded with microchannels which remain either open to atmosphere or pressurized to different positive and negative pressures. We subject these layers to indentation by a rigid cylinder such that in addition to adhesion between the indenter and the adhesive surface, the inner walls of the channels too self-adhere; during retraction of the indenter, these surfaces debond, but at a different load, thus resulting in hysteresis. When these channels are pressurized to different extents, the contact areas of various interfaces vary, so also the resultant hysteresis.

Reusable antifouling viscoelastic adhesive with an elastic skin.

Although the viscoelasticity or tackiness of a pressure-sensitive adhesive gives it strength owing to energy dissipation during peeling, it also renders it nonreusable because of structural changes such as the formation of fibrils, cohesive failure, and fouling. However, an elastic layer has good structural integrity and cohesive strength but low adhesive energy. We demonstrate an effective composite adhesive in which a soft viscoelastic bulk layer is imbedded in a largely elastic thin skin layer.

Peeling off an adhesive layer with spatially varying modulus.

We analyze here displacement controlled peeling of a flexible adherent off a thin layer of elastic adhesive, the elastic modulus of which does not remain uniform but varies periodically along the direction of peeling. Calculation shows that with progressive peeling, the crack front does not propagate continuously at the interface but intermittently with crack arrests and subsequent initiations. The crack gets arrested close to the location of the minimum shear modulus of the layer and initiates again only at a sufficiently large peel off load.

A bioinspired wet/dry microfluidic adhesive for aqueous environments.

A pressure-sensitive, nonreacting and nonfouling adhesive which can perform well both in air and underwater is very desirable because of its potential applications in various settings such as biomedical, marine, and automobile. Taking a clue from nature that many natural adhesive pads have complex structures underneath the outer adhesive layer, we have prepared thin elastic adhesive films with subsurface microstructures using PDMS (poly(dimethylsiloxane)) and investigated their performance underwater. The presence of embedded structure enhances the energy of adhesion considerably both in air and underwater.

Bioinspired design of a hierarchically structured adhesive.

The mechanism by which many creatures such as geckos can run at ease on a vertical wall and yet remain strongly adhered has been linked to hierarchically patterned microstructures: flexible pads, hairs, and subsurface fluidic vessels at their feet. Despite many advances, how these features of different length scales and the associated physical phenomena couple to engender this "smart" adhesive is yet to be understood and mimicked. In this context, we have designed elastomeric films of poly(dimethylsiloxane) embedded with stacks of planar microchannels, curved and straight, and channels with microscopically patterned walls.

Hysteresis of soft joints embedded with fluid-filled microchannels.

Many arthropods are known to achieve dynamic stability during rapid locomotion on rough terrains despite the absence of an elaborate nervous system. While muscle viscoelasticity and its inherent friction have been thought to cause this passive absorption of energy, the role of embedded microstructures in muscles and muscle joints has not yet been investigated. Inspired by the soft and flexible hinge joints present in many of these animals, we have carried out displacement-controlled bending of thin elastic slabs embedded with fluid-filled microchannels.

Three-dimensional multihelical microfluidic mixers for rapid mixing of liquids.

Rapid mixing of liquids is important for most microfluidic applications. However, mixing is slow in conventional micromixers, because, in the absence of turbulence, mixing here occurs by molecular diffusion. Recent experiments show that mixing can be enhanced by generating transient flow resulting in chaotic advection.

Microfluidic adhesion induced by subsurface microstructures.

Natural adhesives in the feet of different arthropods and vertebrates show strong adhesion as well as excellent reusability. Whereas the hierarchical structures on the surface are known to have a substantial effect on adhesion, the role of subsurface structures such as the network of microchannels has not been studied. Inspired by these bioadhesives, we generated elastomeric layers with embedded air- or oil-filled microchannels.

Kink instability of a highly deformable elastic cylinder.

When a soft elastic cylinder is bent beyond a critical radius of curvature, a sharp fold in the form of a kink appears catastrophically at its compressed side while the tensile side remains smooth. The critical radius increases linearly with the diameter of the cylinder but remains independent of its material properties such as modulus; the maximum deflection at the location of the kink depends on both the material and geometric properties of the cylinder. The catastrophic dynamics of evolution of the kink depicts propagation of a shear wave from the location of the kink towards the edges signifying that kinking is an elastic response of the material which results in extreme localization of curvature.

Embedded template-assisted fabrication of complex microchannels in PDMS and design of a microfluidic adhesive.

In this paper, we describe a novel method for fabricating 2-D and 3-D microchannel patterns in a flexible platform of cross-linked poly(dimethylsiloxane) (PDMS). Here, a slender nylon thread formed into different 2-D and 3-D shapes is used as a template that is embedded inside a block of cross-linked PDMS. The cross-linked network is then allowed to swell in a suitable solvent that swells the network selectively but leaves the nylon thread unaltered.