arXiv subjects

Vivek B. Shenoy

Publications and source records attributed to Vivek B. Shenoy.

21 records · Page 2Linked to original sources

arXiv2010

Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene

Using molecular dynamics simulations and first principles calculations, we have studied the structure and mechanical strength of tilt grain boundaries in graphene sheets that arise during CVD growth of graphene on metal substrates. Surprisingly, we find that for tilt boundaries in the vicinity of both the zig-zag and arm-chair orientations, large angle boundaries with a higher density of 5-7 defect pairs are stronger than the low-angle boundaries which are comprised of fewer defects per unit length. Interestingly, the trends in our results cannot be explained by a continuum Griffith-type fracture mechanics criterion, which predicts the opposite trend due to that fact that it does not account for the critical bonds that are responsible for the failure mechanism. We have identified the highly-strained bonds in the 7-member rings that lead to the failure of the sheets, and we have found that large angle boundaries are able to better accommodate the strained 7-rings. Our results provide guidelines for designing growth methods to obtain grain boundary structures that can have strengths close to that of pristine graphene.

cond-mat.mtrl-sci↗

arXiv2010

Low-Reynolds number swimming in gels

Many microorganisms swim through gels, materials with nonzero zero-frequency elastic shear modulus, such as mucus. Biological gels are typically heterogeneous, containing both a structural scaffold (network) and a fluid solvent. We analyze the swimming of an infinite sheet undergoing transverse traveling wave deformations in the "two-fluid" model of a gel, which treats the network and solvent as two coupled elastic and viscous continuum phases. We show that geometric nonlinearities must be incorporated to obtain physically meaningful results. We identify a transition between regimes where the network deforms to follow solvent flows and where the network is stationary. Swimming speeds can be enhanced relative to Newtonian fluids when the network is stationary. Compressibility effects can also enhance swimming velocities. Finally, microscopic details of sheet-network interactions influence the boundary conditions between the sheet and network. The nature of these boundary conditions significantly impacts swimming speeds.

physics.bio-ph↗

arXiv2008

Role of slip between a probe particle and a gel in microrheology

In the technique of microrheology, macroscopic rheological parameters as well as information about local structure are deduced from the behavior of microscopic probe particles under thermal or active forcing. Microrheology requires knowledge of the relation between macroscopic parameters and the force felt by a particle in response to displacements. We investigate this response function for a spherical particle using the two-fluid model, in which the gel is represented by a polymer network coupled to a surrounding solvent via a drag force. We obtain an analytic solution for the response function in the limit of small volume fraction of the polymer network, and neglecting inertial effects. We use no-slip boundary conditions for the solvent at the surface of the sphere. The boundary condition for the network at the surface of the sphere is a kinetic friction law, for which the tangential stress of the network is proportional to relative velocity of the network and the sphere. This boundary condition encompasses both no-slip and frictionless boundary conditions as limits. Far from the sphere there is no relative motion between the solvent and network due to the coupling between them. However, the different boundary conditions on the solvent and network tend to produce different far-field motions. We show that the far field motion and the force on the sphere are controlled by the solvent boundary conditions at high frequency and by the network boundary conditions at low frequency. At low frequencies compression of the network can also affect the force on the sphere. We find the crossover frequencies at which the effects of sliding of the sphere past the polymer network and compression of the gel become important.

cond-mat.soft↗