arXiv subjects

Ashutosh Sharma

Publications and source records attributed to Ashutosh Sharma.

30 records · Page 2Linked to original sources

arXiv2010

Micro-fabrication of Carbon Structures by Pattern Miniaturization in Resorcinol-Formaldehyde Gel

A simple and novel method to fabricate and miniaturize surface and sub-surface micro-structures and micro-patterns in glassy carbon is proposed and demonstrated. An aqueous resorcinol-formaldehyde (RF) sol is employed for micro-molding of the master-pattern to be replicated, followed by controlled drying and pyrolysis of the gel to reproduce an isotropically shrunk replica in carbon. The miniaturized version of the master-pattern thus replicated in carbon is about one order of magnitude smaller than original master by repeating three times the above cycle of molding and drying. The micro-fabrication method proposed will greatly enhance the toolbox for a facile fabrication of a variety of Carbon-MEMS and C-microfluidic devices.

cond-mat.soft↗

arXiv2010

Time-aging time-stress superposition in soft glass under tensile deformation field

We have studied the tensile deformation behaviour of thin films of aging aqueous suspension of Laponite, a model soft glassy material, when subjected to a creep flow field generated by a constant engineering normal stress. Aqueous suspension of Laponite demonstrates aging behaviour wherein it undergoes time dependent enhancement of its elastic modulus as well as its characteristic relaxation time. However, under application of the normal stress, the rate of aging decreases and in the limit of high stress, the aging stops with the suspension now undergoing a plastic deformation. Overall, it is observed that the aging that occurs over short creep times at small normal stresses is same as the aging that occurs over long creep times at large normal stresses. This observation allows us to suggest an aging time - process time - normal stress superposition principle, which can predict rheological behaviour at longer times by carrying out short time tests.

cond-mat.soft↗

arXiv2010

Spinodal Phase Separation in Liquid Films with Quenched Disorder

We study spinodal phase separation in unstable thin liquid films on chemically disordered substrates via simulations of the thin-film equation. The disorder is characterized by immobile patches of varying size and Hamaker constant. The effect of disorder is pronounced in the early stages (amplification of fluctuations), remains during the intermediate stages and vanishes in the late stages (domain growth). These findings are in contrast to the well-known effects of quenched disorder in usual phase-separation processes, viz., the early stages remain undisturbed and domain growth is slowed down in the asymptotic regime. We also address the inverse problem of estimating disorder by thin-film experiments.

cond-mat.soft↗

arXiv2010

Kinetics of Spinodal Phase Separation in Unstable Thin Liquid Films

We study universality in the kinetics of spinodal phase separation in unstable thin liquid films, via simulations of the thin film equation. It is shown that in addition to morphology and free energy,the number density of local maxima in the film profile can also be used to identify the early, intermediate and late stages of spinodal phase separation. A universal curve between the number density of local maxima and rescaled time describes the kinetics of early stage in d = 2, 3. The Lifshitz-Slyozov exponent of -1/3 describes the kinetics of the late stage in d = 2 even in the absence of coexisting equilibrium phases.

cond-mat.soft↗

arXiv2009

Electric field Induced Patterns in Soft Visco-elastic films: From Long Waves of Viscous Liquids to Short Waves of Elastic Solids

We show that the electric field driven surface instability of visco-elastic films has two distinct regimes: (1) The visco-elastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as ~ 4 x film thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus.

cond-mat.soft↗

arXiv2009

Tensile Deformation and Failure of Thin Films of Aging Laponite Suspension

In this paper we study deformation, failure and breakage of visco-elastic thin films of aging laponite suspension under tensile deformation field. Aqueous suspension of laponite is known to undergo waiting time dependent evolution of its micro-structure, also known as aging, which is accompanied by an increase in the elastic modulus and relaxation time. In the velocity controlled tensile deformation experiments, we observed that the dependence of force and dissipated energy on velocity and initial thickness of the film is intermediate to a Newtonian fluid and a yield stress fluid. For a fixed waiting time, strain at break and dissipated energy increased with velocity, but decreased with initial thickness. With increase in age, strain at break and dissipated energy showed a decrease suggesting enhanced brittle behavior with increase in waiting time, which may be caused by restricted relaxation modes due to aging. In a force controlled mode, decrease in strain at failure at higher age also suggested enhanced brittleness with increase in waiting time. Remarkably, the constant force tensile deformation data up to the point of failure showed experimental time- aging time superposition that gave an independent estimation of relaxation time and elastic modulus dependence on age.

cond-mat.soft↗

arXiv2006

Contact Instability in Adhesion and Debonding of Thin Elastic Films

Based on experiments and 3-D simulations, we show that a soft elastic film during adhesion and debonding from a rigid flat surface undergoes morphological transitions to pillars, labyrinths and cavities, all of which have the same lateral pattern length scale, close to λ/H ~ 3 for thick films, H > 1 micrometer. The linear stability analysis and experiments show a new thin film regime where λ/H \approx 3+ 2 (γ/3 μH)^(1/4) (γis surface tension, μis shear modulus) because of significant surface energy penalty (for example, λ/H = 6 for H = 200 nm; μ= 1MPa).

cond-mat.soft↗

arXiv2003

Patterns, Forces and Metastable Pathways in Debonding of Elastic Films

The letter resolves several intriguing and fundamental aspects of debonding at soft interfaces, including the formation and persistence of regularly arranged nanocavities and bridges, ``adhesion-debonding hysteresis'', and vastly lower adhesive strengths compared to the absence of pattern formation. The analysis shows the hysteresis to be caused by an energy barrier that separates the metastable patterned configurations during withdrawal, and the debonded state. The metastable morphological pathways involving cavitation and peeling of contact zones engender substantially lower debonding forces.

cond-mat.soft↗

arXiv2002

Dewetting of Glassy Polymer Films

Dynamics and morphology of hole growth in a film of power hardening viscoplastic solid (yield stress ~ [strain-rate]^n) is investigated. At short-times the growth is exponential and depends on the initial hole size. At long-times, for n > 1/3, the growth is exponential with a different exponent. However, for n < 1/3, the hole growth slows; the hole radius approaches an asymptotic value as time tends to infinity. The rim shape is highly asymmetric, the height of which has a power law dependence on the hole radius (exponent close to unity for 0.25 < n < 0.4). The above results explain recent intriguing experiments of Reiter, Phys. Rev. Lett, 87, 186101 (2001).

cond-mat.soft↗

arXiv2001

Pattern Formation in a Substrate--Contactor System with Two Interacting Incompressible Elastic Films

The surface stability of two interacting (for example, by van der Waals forces) incompressible thin films, one bonded to a substrate and the other to a contactor, is studied extending the work of Shenoy and Sharma, Physical Review Letters 18, 119--122 (2001). The analysis indicates that the wavelength of the instability depends strongly on the shear moduli and thicknesses of the films but not on the nature and magnitude of the interaction. When the films have equal shear moduli, the wavelength of the instability has an intermediate value between the wavelengths of the instabilities had each of the films been interacting with rigid contactors. On the other hand, if the films have different shear moduli but equal thicknesses, then the wavelength of the instability is identical to that had the films been interacting with rigid contactors. In the more general case when the two films have different shear moduli and thicknesses, the nature of the critical wavelength is more complex. When ratio of the shear moduli of the contacting film to that of the film bonded to the substrate exceeds 5.32, the wavelength of the of the instability jumps from the value close to that determined by the thickness of the film bonded to the substrate to that of the contacting film, as the thickness of the contacting film is increased.

cond-mat.soft↗

arXiv2000

Stability of a Thin Solid Film with Interactions

We investigate the question of stability of a solid thin film which experiences external interactions such as van der Waals forces from a contacting surface or forces from an external electric field. Both perfectly elastic and viscoelastic material behaviours are considered in linear stability analysis performed here. These analyses indicate that for sufficiently soft (shear modulus between 1 and 10 MPa) and nearly incompressible films (Poisson's ratio close to 0.5), bifurcations are possible, i.e., the surface of the film becomes non-planar. The modes of bifurcation and rates of growth of perturbations are determined as a function of material parameters. The results of this study are of significance in understanding the adhesive properties between a soft material (such as rubber) and a comparatively rigid solid (such as steel), and the behaviour of soft solid films in an electric field.

cond-mat.mtrl-sci↗

arXiv1999

Intermittency in Inhomogeneous Coupled Map Lattices

We study the phenomenon of intermittency in inhomogeneous lattices of coupled map where inhomogeneity appears in the form of different values of map parameters at adjacent sites.The system exhibits spatiotemporal intermittency in various regions of parameter space.We identify the types of co-dimension two bifurcations which give rise to spatio-temporal or purely spatial intermittency and study the mechanism in one case, that of synchronised fixed point Power law distributions of laminar lengths is observed in vicinity of such co-dim 2 points and the exponents fall in three ranges for three different types of bifurcations. Three of the exponents seen in this model show good agreement with those observed in fluid experiments with quasi-one dimensional geometries.

chao-dyn↗