Srikanth Vedantam

We present a dissipative particle dynamics (DPD) study of the deformation of capsules in microchannels. The strain in the membrane during this deformation causes the formation of temporary pores, which is termed mechanoporation. Mechanoporation is being considered as a means by which intracellular delivery of a broad range of cargo can be facilitated. In this work, we examine the strain distribution on the capsule membrane during transport of the capsule in converging-diverging microchannels of different constriction widths. The pore density is correlated to the strain in the membrane. We find that the highest strains and, consequently, the highest pore densities occur at intermediate channel widths. This occurs due to a competition of the bending of the membrane and fluid shear stresses in the flow.

When a Unidirectional fiber reinforced polymer (UDFRP) composite is subjected to transverse loading, there is spatial variation of stresses in the constituents. The failure in matrix initiates at the location of maximum stress. Stress distribution and failure initiation in constituents of UDFRP composites is usually studied through finite element (FE) analysis of representative volume element (RVE) which is computationally expensive and time consuming. The present study proposes an analytical model through which stress variation and failure initiation in the constituents of UDFRP composite can be obtained in simple and reliable way and it can be readily used in designing. For this model, RVE is idealized in the form of springs arranged in parallel and series. These springs represent the stiffness of constituents (fiber and matrix). The results of analytical model are compared with FE simulations and good agreement is observed. Influence of fiber volume fraction on failure initiation of UDFRP composites is also studied through FE analysis of RVE and analytical model.

This paper aims to estimate the enhancement in the energy absorption characteristics of the glass fiber reinforced composites (GFRP) by embedding prestrained pseudo-elastic shape memory alloy (SMA) that was used as a secondary reinforcement. The pseudo-elastic SMA (PESMA) embedded were in the form of wires and have an equiatomic composition (i.e., 50%–50%) of nickel (Ni) and titanium (Ti). These specimens are fabricated using a vacuum-assisted resin infusion process. The estimation is done for the GFRP and SMA/GFRP specimens at four different impact velocities (65, 75, 85, and 103 m/s) using a gas-gun impact set-up. At all different impact velocities, the failure modes change as we switch from GFRP to SMA/GFRP specimen. In the SMA/GFRP specimen, the failure mode changed from delamination in the primary region to SMA-pull out and SMA deformation. This leads to an increase in the ballistic limit. It is observed that energy absorbed by SMA/GFRP specimens is higher than the GFRP specimens subjected to the same levels of impact energy. To understand the damping capabilities of SMA embedment, vibration signals are captured, and the damping ratio is calculated. SMA dampens the vibrations imparted by the projectile to the specimen. The damping ratio of the SMA/GFRP specimens is higher than the GFRP specimens. The damping effect is more prominent below the ballistic limit when the projectile got rebounded (65 m/s).

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.

Resin Film Infusion (RFI) process is used for manufacturing large and thick unidirectional fiber reinforced polymer (UD FRP) composite structures for load bearing applications. Designing these load bearing structures requires the knowledge of effective elastic and strength characteristics of these composites. In this study, strength and stiffness properties of UD FRP manufactured through RFI process are predicted using a micromechanical modeling approach. A 3D representative volume element (3DRVE) with hexagonal array of fibers is used and all nine elastic constants of UD FRP composite are predicted. Failure models for fiber and resin are used to predict longitudinal tension/compression, transverse tension/compression and longitudinal shear strengths. Experiments were conducted on UD FRP composite manufactured through RFI process to obtain strength and stiffness properties. Results of experiment and simulations are compared and results validated.

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.

The shape memory effect observed in many alloys arises due to stress induced transformation between variants of the martensitic phase. It is difficult to study this process in detail using continuum approaches and particle based methods are eminently more suitable. In this work, we study detwinning, which is the transformation between martensite variants due to applied stress using a novel discrete particle model. The approach uses a novel multibody interparticle interaction defined directly using a non-convex free energy potential pertinent to such material behavior. This model is able to describe simultaneously occurring multiple microscale events during the detwinning process: nucleation and propagation of ledges along twin boundary. Due to these underlying microscale events the plateau region of stress–strain response shows a jerky nature. The effect of temperature and morphology on the stress–strain behavior of the self-accommodated martensite microstructure is studied in detail. From the simulations, we identify the morphological features affecting the transformation stress for detwinning. The critical parameters are found to be the length and number of twin and macro-twin boundaries and the number of mobile ledges.