Srikanth Vedantam

In this paper, we investigate the dynamics of a tethered flexible filament due to fluid flow inside a microchannel. We use the finite sized dissipative particle dynamics (FDPD) approach to model this problem. The flexible filament is modeled as a bead-spring system with both extensional and flexural rigidity. The influence of flow rate and bending stiffness on the filament dynamics is studied in terms of the different conformational modes obtained. The competing effects of the hydrodynamic force and elastic force in the presence of Brownian thermal effects of comparable order influence the mode shapes of the filament. The dynamics of the filament motions are further analyzed using proper orthogonal decomposition. An important consequence of the dynamics of the filament is that it causes cross-flow in the micro-channel, which could potentially be exploited in micro-mixing and pumping applications. The cross stream fluid transport is observed to be more pronounced for higher bending stiffness.

Dissipative particle dynamics (DPD) is an efficient, particle based mesoscopic numerical scheme to simulate dynamics of complex fluids and micro-flows, with spatio-temporal scales in the range of micrometers and microseconds. While the traditional DPD method treated particles as point masses, a modified DPD scheme was introduced recently [W. Pan, I.V. Pivkin, G.E. Karniadakis, Single-particle hydrodynamics in DPD: a new formulation, Europhysics Letters 84 (2008) 10012] by including transverse forces between finite sized particles in addition to the central forces of the standard DPD.The capability of a DPD scheme to solve confined wall bounded flows, depends on its ability to model the flow boundaries and effectively impose the classical no-slip boundary condition. Previous simulations with the modified DPD scheme used boundary conditions from the traditional DPD schemes, resorting to the velocity reversal of re-inserted particles which cross the solid wall. In the present work, a new method is proposed to impose no-slip or tunable slip boundary condition by controlling the non-central dissipative components in the modified DPD scheme. The solid wall is modeled in such a way that the fluid particles feel the presence of a continuous wall rather than a few discrete frozen particles as in conventional wall models. The fluid particles interact with the walls using a modified central repulsive potential to reduce the spurious density fluctuations. Several different benchmark problems (Poiseuille flow, lid-driven cavity and flow past circular cylinder) were solved using the new approach to demonstrate its validity. © 2012 Elsevier Inc.

Grain rotation has been found to be an important mechanism of microstructure evolution in addition to curvature-driven grain boundary migration in nanocrystalline materials. Grains coalesce due to rotation and this provides an additional mechanism of grain growth. We show that for growth by coupled migration and rotation coalescence the average grain size grows as R ~ t1/3 indicating that the coupled problem is surprisingly more stable to grain growth. We then present an appropriate rotationally invariant multiphase field model which accounts for grain rotation to corroborate the theoretical result.

We investigate the effect of the Schmidt number (Sc) on phase separation dynamics of two immiscible fluids in a two-dimensional periodic box using dissipative particle dynamics. The range of Sc investigated spans liquid-liquid separation processes. Phase separation is characterized by a domain size r(t), which typically follows a power law tβ in time t, where β is a characteristic exponent corresponding to the coarsening mechanism at play. The phase separation dynamics is studied for strongly (deep quench) separating mixtures. We consider cases of critical (φ ∼0.5) and off-critical (φ < 0.5) mixtures of fluids A and B for both ScA = ScB and ScA ≠ ScB. In all cases, the growth dynamics slow down with the increasing Schmidt number of either fluid. We observe the power law exponent β = 0.5 for symmetric (ScA = ScB) critical mixtures and β = 0.33 for all other cases. However, for off-critical mixtures, the exponent is 0.33 irrespective of the Schmidt number ratio of the two fluids. We explain these results from an analysis of the competition between diffusive effects vis-á-vis dynamical forces.

Understanding the interactions of Σ3n boundaries and the triple junction character is important in the context of grain boundary engineering in cubic materials. In a triple junction, when two boundaries are Σ3 and Σ9, the third boundary is either a Σ3 or a Σ27 as per the Σ-combination rule. In the present work, the role of the misorientation axis of the interacting Σ boundaries on the character of the third boundary is systematically studied. The calculated probabilities of occurrences of Σ3, Σ9 and Σ27 boundaries are correlated with the experimental triple junction distributions in Ni and Ni based superalloys.

In this work we investigated the friction and wear properties of Fe/SiC/graphite hybrid composites using a sub-scale dynamometer disk brake testing system. Two particle size ranges (1 - 30 μm and 150 - 180 μm) and three particle volume fractions (10%, 15% and 20%) of SiC were considered. The sliding speed conditions considered in this study (25-35m/s) were comparable to that experienced by brake materials in high speed braking applications in aircrafts, race car and high speed trains. We examined the effect of coating the SiC particles with BaSO4 to improve interfacial properties and prevent potential undesirable interfacial reactions. The wear loss was found to decrease with increasing volume fraction of SiC for all particulate sizes. At low sliding speeds the composites with large particle sizes and high volume fractions were found to be more effective in controlling wear. On the other hand, at higher sliding speeds the high volume fraction composites were found to be more effective in controlling wear for all particle sizes. This is attributed to a transition in the wear mechanism at higher sliding speeds. © 2013 Elsevier B.V.

In this paper, we study equilibrium three-dimensional shapes of drops on hysteretic surfaces. We develop a function coupled with the publicly available surface energy minimization code Surface Evolver to handle contact angle hysteresis. The function incorporates a model for the mobility of the triple line into Surface Evolver. The only inputs to the model are the advancing and receding contact angles of the surface. We demonstrate this model's versatility by studying three problems in which parts of the triple line advance while other parts either recede or remain stationary. The first problem focuses on the three-dimensional shape of a static pendant drop on a vertical surface. We predict the finite drop volume when impending sliding motion is observed. In the second problem, we examine the equilibrium shapes of coalescing sessile drops on hysteretic surfaces. Finally, we study coalescing puddles in which gravity plays a leading role in determining the equilibrium puddle shape along with hysteresis. © 2012 Springer-Verlag.

Thin deformable membranes are encountered in a number of microfluidics-based applications. These are often employed for enhancing sorting, mixing, cross-diffusion transport, etc. Microfluidic systems with deformable membranes can be better understood by employing simple models and efficient computational procedures. In this paper, we present a dissipative particle dynamics model to simulate the interaction between a deformable membrane and fluid flow in a two-dimensional microchannel. The membrane is modeled as a bead-spring system with both extensional and torsional springs to simulate extensional stiffness and bending rigidity, respectively. By performing detailed simulations on a membrane pinned at both ends and oriented parallel to the flow, we observe different steady state conformations. These membrane deflections are found to be relatively large for low bending stiffnesses and small for high stiffnesses. The membrane was found to exhibit a simple bowing out mode for high stiffness values and more complex conformations at lower stiffnesses.

In this work, we numerically study a new means of manipulating single DNA chains in microchannels. The method is based on the effect of finite slip at hydrophobic walls on the hydrodynamics and, consequently, on the dynamics of the DNA in microchannels. We use dissipative particle dynamics to study DNA transport as a function of chain length and the Reynolds number in two dimensional parallel plate channels. We show how an asymmetric velocity profile in a channel with hydrophobic and hydrophilic walls can be used to manipulate the location of the DNA molecules. Using this effect, we propose a simple arrangement of hydrophobic and hydrophilic strips which can be exploited to separate long and short DNA chains. This journal is © the Partner Organisations 2014.

In this paper we studied the tribological behavior of iron matrix composites at high sliding speeds (25-35. m/s) typical of aircraft braking conditions. We developed two types of Fe matrix composites with different elastic modulus reinforcements: silica (71. GPa) and mullite (143. GPa) particulates using powder metallurgy. Two different size ranges: large (150 - 250 μm) and small sizes (1 - 10 μm) and a range of volume fractions of the particulates were also considered. The dry sliding wear and braking performance of the composites were investigated using a sub-scale disc braking dynamometer. The wear tests of the composites show that large size and high volume fraction of reinforcement particles provides better wear resistance and braking performance at high sliding speed conditions (25. m/s-35. m/s) for both Fe/silica composites and Fe/mullite composites. Significantly, Fe/mullite composites at lower volume fractions showed greater wear resistance than the Fe/silica composites due to the higher elastic modulus of the mullite particles. A wear track examination of composites showed that different wear mechanisms were operative at the different speeds. Our results indicate that composites with a high volume fraction of large sized reinforcement particles of high elastic modulus are to be preferred for braking performance and low wear loss at high sliding speed applications. © 2013 Elsevier B.V.