arXiv subjects

Manoj K. Chaudhury

Publications and source records attributed to Manoj K. Chaudhury.

22 records · Page 2Linked to original sources

arXiv2014

Direct Measurement of the Surface Tension of a Soft Elastic Hydrogel: Exploration of Elasto-Capillary Instability in Adhesion

An adhesively stressed thin film of a soft hydrogel confined between two rigid flat substrates auto-roughens with its dominant wavelength exhibiting pronounced dependence on the film thickness (H). A linear stability analysis confirmed that this long wavelength instability (~7H) is due to an elasto-capillary 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 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 elasto-capillarity 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 is linear with H, which modifies the conventional stress to pull-off a rigid flat object from a very soft film by a multiplicative factor. 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 non-trivial consequences in the adhesive delamination of very thin and/or soft elastic films via self-generated cracks.

cond-mat.soft↗

arXiv2014

Surface Folding Induced Attraction and Motion of Particles in a Soft Elastic Gel: Cooperative Effects of Surface Tension, Elasticity and Gravity

We report, for the first time, some experimental observations regarding a new type of long range interaction between rigid particles that prevails when they are suspended in an soft elastic gel. A denser particle submerges itself to a considerable depth inside the gel and becomes buoyant by balancing its weight against the elastic force exerted by the surrounding medium. By virtue of a large elastic-capillary length, the surface of the gel wraps around the particle and closes to create a line singularity connecting the particle to the free surface of the gel. Substantial amount of tensile strain is thus developed in the gel network parallel to the free surface that penetrates to a significant depth inside the gel. The field of this tensile strain is rather long range owing to a large correlation length and strong enough to pull two submerged particles into contact. The particles move towards each other with an effective force following an inverse linear distance law. When more monomers or dimers of the particles are released inside the gel, they orient rather freely inside the capsules they are in, and attract each other to form close packed clusters. Eventually, these clusters themselves interact and coalesce. This is an emergent phenomenon in which the gravity, the capillarity and the elasticity work in tandem to create a long range interaction. We also present the results of a related experiment, in which a particle suspended inside a thickness graded gel moves accompanied by the continuous folding and the relaxation of the surface of the gel.

cond-mat.soft↗

arXiv2011

Rolling motion of a rigid sphere on a structured rubber substrate aided by a random noise and an external bias

We study the rolling motion of a small solid sphere on a surface patterned rubber substrate in an external field with and without a noise. In the absence of the noise, the ball does not move below a threshold force above which it accelerates sub-linearly with the applied field. In the presence of a random noise, the ball exhibits a stochastic rolling with a net drift, the velocity of which increases non-linearly with the strength of the noise (K). From the evolution of the displacement distribution, it is evident that the rolling is controlled by a Coulomb like friction at a very low velocity, a super-linear friction at an intermediate velocity and a linear kinematic friction at a large velocity. This transition from a non-linear to a linear friction control of motion can be discerned from another experiment in which the ball is subjected to a small amplitude periodic asymmetric vibration in conjunction with a random noise. Here, as opposed to that of a fixed external force, the rolling velocity decreases with the increase of the strength of the noise suggesting a progressive fluidization of the interface. Furthermore, the ball exhibits a flow reversal with the increase of the amplitude of the asymmetric vibration, which is indicative of a profile of friction that first descends and then ascends with velocity. An approximate non-linear friction model with the non-linearity decreasing with K is somewhat consistent with the evolution of the displacement fluctuation that also explains the sigmoidal variation of the drift velocity with the strength of the noise. This research sets the stage for studying friction in a new way, in which it is submitted to a noise and then its dynamic response is studied using the tools of statistical mechanics.

cond-mat.stat-mech↗

arXiv2007

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. While, 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 which are critically confined. We have presented two different experiments for controlling the degree of confinement: by pre-stretching the film and by adjusting the contact width between the film and the plate.

cond-mat.soft↗