Animangsu Ghatak

Peeling from a biomimetically patterned thin elastic film

Inspired by the observation that many naturally occurring adhesives arise as textured thin films, we consider the displacement–controlled peeling of a flexible plate from an incision–patterned thin adhesive elastic layer. We find that crack initiation from an incision on the film occurs at a load much higher than that required to propagate it on a smooth adhesive surface; multiple incisions thus cause the crack to propagate intermittently. Microscopically, this mode of crack initiation and propagation in geometrically confined thin adhesive films is related to the nucleation of cavitation bubbles behind the incision which must grow and coalesce before a viable crack propagates. Our theoretical analysis allows us to rationalize these experimental observations qualitatively and quantitatively and suggests a simple design criterion for increasing the interfacial fracture toughness of adhesive films.

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. While these are planar microchannels, here we show that three-dimensional orientations of fluidic vessels and channels can enhance significantly mixing of liquids. In particular, we present a novel, multihelical microchannel system built in soft gels, for which the helix angle, helix radius, axial length, and even the asymmetry of the channel cross section are easily tailored to achieve the desired mixing. Mixing efficiency increases with helix angle and asymmetry of channel cross section, which leads to orders of magnitude reduction in mixing length over conventional mixers. This new scheme of generating 3D microchannels will help in miniaturization of devices, process intensification, and generation of multifunctional process units for microfluidic applications.

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. The composite layer is able to meet the conflicting demands of the high peel strength comparable to the viscoelastic core and the structural integrity, reusability, and antifouling properties of the elastic skin. Our model adhesive is made of poly(dimethylsiloxane), where its core and skin are created by varying the cross-linking percentage from 2 to 10%.

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. When such a confined film is subjected to a continually increasing normal displacement, the morphological patterns evolve into cracks, which, in turn, govern the adhesive fracture behavior of the interface. While, in general, the thickness provides the relevant length scale underlying the well-known Griffith-Kendall criterion of debonding of a rigid disc from a confined film, it is modified non-trivially by the elasto-capillary number for an ultra-soft film. Depending upon the degree of confinement and the spatial distribution of external stress, various analogs of the canonical instability patterns in liquid systems can also be reproduced with thin confined elastic films.Graphical abstract

Confinement-induced instability of thin elastic film.

A confined incompressible elastic film does not deform uniformly when subjected to adhesive interfacial stresses but with undulations which have a characteristic wavelength scaling linearly with the thickness of the film. In the classical peel geometry, undulations appear along the contact line below a critical film thickness or below a critical curvature of the plate. Perturbation analysis of the stress equilibrium equations shows that for a critically confined film the total excess energy indeed attains a minimum for a finite amplitude of the perturbations which grow with further increase in the confinement.

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. Furthermore, filling the channels with liquid of suitable surface tension modifies the internal stress profile, resulting into significant enhancement in adhesive performance. As this increase in adhesion is mediated by mechanics and not by surface chemistry, the presence of water does not alter its performance much. For the same reason, this adhesion mechanism works with both hydrophobic and hydrophilic surfaces. The adhesive can be reused because of its elastic surface. Moreover, unlike many other present-day adhesives, its performance does not decrease with time.

Microchannel Induced Surface Bulging of a Soft Elastomeric Layer

When a wetting liquid fills in microchannels embedded inside a thin elastomeric layer, the surface of the layer does not remain smooth but bulges out in the vicinity of the channels. The height of the bulge depends on the deformability of the layer and the surface tension of liquid; in addition, it depends also on the vertical location of the channel from the surface of the layer and the channel diameter. While, for liquids of low viscosity ∼500 cP bulging occurs instantaneously, for liquids of high viscosity ∼4000 cP, it occurs over a period of time. Concomitant to bulging of the layer, the cross section of the channel alters too in its shape and size suggesting that the elastic energy penalty associated with the bulging of the layer is supplied by the interfacial energy of the liquid–air and liquid–solid interfaces. Local bulging of the layer alters also its local deformability which is demonstrated by contact mechanics experiments: indentation with a spherical indenter yields a non-circular contact area, the shape and size of which vary with the depth of indentation. Thus, sub-surface microchannels can be suitably used for generating surface patterns on the elastomer and also for modulating its modulus.

Critical Confinement and Elastic Instability in Thin Solid Films

When a flexible plate is peeled off a thin and soft elastic film bonded to a rigid support, uniformly spaced fingering patterns develop along their line of contact. Although the wavelength of these patterns depends only on the thickness of the film, their amplitude varies with all material and geometric properties of the film and that of the adhering plate. Here we have analyzed this instability by the regular perturbation technique to obtain the excess deformations of the film over and above the base quantities. Furthermore, by calculating the excess energy of the system, we have shown that these excess deformations, associated with the instability, occur for films that are critically confined. We have presented two different experiments for controlling the degree of confinement: by prestretching the film and by adjusting the contact width between the film and the plate.

A kinetic approach to study the hydrolytic stability of polymer–metal adhesion

A polystyrene-aluminum joint, the adhesion of which was promoted by a silane coupling agent, was examined by fracturing the interface under water at different loads and temperatures. X-ray photoelectron spectroscopy (XPS) analysis of the fractured surfaces showed that the locus of failure was mainly interfacial at low loads, but it gradually moves away from the interface at higher loads. This movement of the failure locus reflects a transition of the mechanism of interfacial debonding from the hydrolysis of the siloxane bonds to the cleavage of the main polymer chains. A rate-dependent bond failure model qualitatively describes the above process, in which the activation energy of bond dissociation is assumed to be a time-dependent parameter.

Estimation of solid–liquid interfacial tension using curved surface of a soft solid

Significance Unlike liquids, for most solids, surface-tension-induced deformation is insignificantly small and practically negligible. It is unclear if, similar to liquids, the surface tension and surface energy of solids, e.g. cross-linked rubber-like materials, are numerically identical. Previous methods involving such materials allowed only indirect and somewhat inaccurate estimation of the surface tension and therefore could not unambiguously resolve the above question. Here we have attempted to address it by using a solid having an intrinsic curvature. When such a surface is contacted, a liquid curvature of the interface, coupled with the interfacial tension, causes large bulging deflection, analysis of which yields direct and accurate estimation of the interfacial tension, even for relatively stiffer solids. 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. In essence, we have used the inner surface of tiny cylindrical channels embedded inside a soft elastomeric film for sensing the effect of the interfacial tension. When a liquid is inserted into the channel, because of wetting-induced alteration in interfacial tension, its thin wall deflects considerably; the deflection is measured with an optical profilometer and analyzed using the Föppl–von Kármán equation. We have used several liquids and cross-linked poly(dimethylsiloxane) as the solid to show that the estimated values of the solid–liquid interfacial tension matches with the corresponding solid–liquid interfacial energy reasonably well.