Ethayaraja Mani

We report a numerical study on the collective dynamics of self-propelling and circle-swimming Lennard-Jones (LJ) particles in two dimensions using Brownian dynamics simulations. We investigate the combined role of attraction, self-propulsion and rotation in their phase behavior. At a low rotational speed, the system shows re-entrant phase behavior as a function of self-propulsion similar to active Brownian particles (ABPs). Increasing the rotational speed shifts the point of re-entrance or makes it disappear depending on the attractive strength. Although active rotation is known to suppress motility induced phase separation, the presence of attractive interactions reduces this effect.

A binary mixture of oppositely charged colloids can be used to stabilize water-in-oil or oil-in-water emulsions. A Monte Carlo simulation study to address the effect of charge ratio of colloids on the stability of Pickering emulsions is presented. The colloidal particles at the interface are modeled as aligned dipolar hard spheres, with attractive interaction between unlike-charged and repulsive interaction between like-charged particles. The optimum composition (fraction of positively charged particles) required for the stabilization corresponds to a minimum in the interaction energy per particle. In addition, for each charge ratio, there is a range of compositions where emulsions can be stabilized. The structural arrangement of particles or the pattern formation at the emulsion interface is strongly influenced by the charge ratio. We find well-mixed isotropic, square, and hexagonal arrangements of particles on the emulsion surface for different compositions at a given charge ratio. The distribution of coordination numbers is calculated to characterize structural features. The simulation study is useful for the rational design of Pickering emulsifications wherein oppositely charged colloids are used, and for the control of pattern formation that can be useful for the synthesis of colloidosomes and porous shells derived thereof.

Unlike in thermodynamic systems, phase separation can occur without a thermodynamic driving force in active systems. How phase separation of purely hydrodynamic origin proceeds is an intriguing physical question. To this end, we study the phase separation of a binary mixture of oppositely rotating disks in a two-dimensional (2D) viscous fluid at an athermal condition by hydrodynamic simulations, focusing on the inertia effect. At symmetric and off-symmetric compositions, phase separation forms the oppositely flowing bands and a circular rotating droplet in the disordered matrix phase. In both cases, phase separation creates the largest structure directly from a chaotic state without gradual domain coarsening, unlike in the thermodynamic and corresponding dry rotor mixtures. We show that this unusual behaviour results from the nonlinear convective acceleration, i.e., the inverse cascade phenomena characteristic of 2D turbulence. Our finding reveals nontrivial nonlinear hydrodynamic effects on the self-organisation of active/driven particles in a fluid.

We carry out coarse-grained Brownian dynamics simulations of shearing flow of a colloidal suspension bridged by telechelic polymers with “sticky” end groups and vary sticker strength ϵ over a range from 3 to 12 in units of kBT, motivated by an interest in simulating the rheology of latex paints. The most extensive results are obtained for dumbbells, but the trends are confirmed for 3-bead trumbbells and chains of up to 11 beads. The numbers of colloids and of polymers are also varied over a wide range to confirm trends established for smaller, more computationally affordable, systems. The dynamics are the result of an interplay of the shear rate and three different times scales: the time τBridge for a sticker on a bridging chain to be released from a particle surface, which scales as exp(0.77ϵ), the time for the polymer chain to relax, τR, which scales as the square of polymer chain length, and the time τD for a colloid to diffuse a distance comparable to its own radius, R, which scales as R3. The scalings of the bridge-to-loop and loop-to-bridge times namely τBL ∝ exp (0.75ϵ) and τLB ∝ exp (0.71ϵ), are similar to those of τBridge, for ϵ values above around 5 kBT, because of the relatively short chains considered here (i.e., 60 Kuhn steps). However, τR becomes more dominant for longer chains, as shown by Travitz and Larson. The zero-shear viscosity η0 is estimated from the Green-Kubo relation, and found to scale as exp (0.69ϵ), similar to that of τBridge. A weak influence of η0 on τD is observed, with the influence expected to become stronger when τD becomes larger, as shown previously by Wang and Larson. At shear rates in the nonlinear regime, shear-thinning is found with exponents ≈ −0.10 to −0.60, and the first normal stress difference is positive, consistent with some of the experimental data of Chatterjee et al. on model latex paint formulations. The weakness of the shear thinning, relative to that of hydrophobically modified ethoxylated urethane (HEUR) solutions without colloids, is likely due to the observed insensitivity of the loop-to-bridge and bridge-to-loop transition times to the imposed shear rate. This preliminary study provides the first mesoscale simulations of these suspensions, useful for assessing and improving both more accurate multi-scale models and eventually constitutive equations for these complex suspensions.

Non-spherical patchy particles are potential candidates as building blocks for the design of target colloidal structures via spontaneous self-organization. We report a facile scheme to synthesize non-spherical particles with patchy electrostatic interactions. In this method, charged spherical latex particles such as polystyrene (PS) are deformed unequally at an oil-water interface due to heating and partial swelling. The spherical particles then evolve into non-spherical shapes such as 'acorn-like' and 'idly-like'. We explain the mechanism of differential deformation by comparing the heat of viscous dissipation and the interfacial energies. Furthermore, if oppositely charged additives such as the cetyltrimethylammonium bromide (CTAB) surfactant or silica nanoparticles are present in water (subphase), electrostatic attraction leads to adsorption of these species on the PS surface exposed to water. As a result, one side of the particles is selectively functionalized, while the other side remains unaltered. As the latex particles are negatively charged initially, this method yields particles that are non-spherical in shape and with negative charges on one side and positive charges on the other side. The degree of shape deformation and patch coverage can be varied by choosing different surface active additives. We extend this approach to curved interfaces and demonstrate a high throughput emulsion based approach for the synthesis of such particles. Self-assembly of these particles shows interesting structures such as linear, branched polymeric or worm-like chains and micelle-like spherical aggregates. These shape anisotropic particles with orientation specific interactions that mimic bio-macromolecular systems can be further explored for self-assembly into hierarchical mesoscale structures.

In equilibrium, colloidal suspensions governed by short-range attractive and long-range repulsive interactions form thermodynamically stable clusters. Using Brownian dynamics computer simulations, we investigate how this equilibrium clustering is affected when such particles are self-propelled. We find that the clustering process is stable under self-propulsion. For the range of interaction parameters studied and at low particle density, the cluster size increases with the speed of self-propulsion (activity) and for higher activity the cluster size decreases, showing a nonmonotonic variation of cluster size with activity. This clustering behavior is distinct from the pure kinetic (or motility-induced) clustering of self-propelling particles which is observed at significantly higher activities and densities. We present an equilibrium model incorporating the effect of activity as activity-induced attraction and repulsion by imposing that the strength of these interactions depend on activity superlinearly. The model explains the cluster size dependence of activity obtained from simulations semiquantitatively. Our predictions are verifiable in experiments on interacting synthetic colloidal microswimmers.

Standard model and/or QCD broadly envisage operation of a symmetry breaking in creation of mass [1]. Likewise, any phase transition brought about by a critical transition refer to operation of a symmetry breaking [2,3]. Such aspects of symmetry breaking directly follows from Noether's theorem and further implies that in principle, non-equlibrium self-assembly could emulate under correct electrochemical conditions creation of ‘matter’. In this work, we test this earlier conjecture of ours and show with experimental evidence how space-time symmetry breaking emulated by electrochemical realization of a ‘Time-Gel’ in the lab brought about by a critical phenomenon can be explored to demonstrate, at least in principle, origin of matter. The ‘Time-Gel’ is realized by electrochemical induction of a critical phenomenon of ‘gelation’ of discrete molecular precursors, where upon with temporally controlled charge injection/abstraction Q (as an indicator of temporal information) we evidence direct creation of ‘Time-Gel's (a polymer-colloid mixture) ‘mass’ (volume fraction, Φg) from a mass ‘null’ state. ‘Time-Gel's volume fraction (Φg) acts as a descriptor of spatio-temporal information. We observe further the descriptor of temporal information (Q) varies with volume fraction of gel formed (Φg) manifest as spatio-temporal fluctuation with a relation Q∝ Φgn. The order of n, is 5, and is obtained experimentally. Such an exponent in the power law, implies divergence in creation of matter. The above observation implicit in our finding is consistent to existing knowledge of divergent creation of matter as a direct consequence of breaking of symmetry under conditions following directly from Noether's theorem. In short, the present proposition prompted by experimental findings centers around the idea of symmetry breaking, where an experimental realization of a ‘time-gel’ model shows how space-time and charge might have led to the formation of matter under conditions away from equilibrium. ‘Time-Gel’ may thus act as an alternative model based on Chemical Science to explain and envisage the emergence of matter as a direct consequence of Noether's theorem.

Amyloid β (Aβ40) is a class of amyloidogenic proteins known to aggregate into a fibrillar network. The rate of aggregation and fibril yield is sensitive to external energy input, such as shear. In this work, simple shear and shaking experiments are performed on Aβ40 solution using a Couette cell and an orbital shaker, respectively. Experiments show that, under uniform shear, both the mass of fibrils and aggregation rate increase with the shear rate. In the case of orbital shaking, the lag time decreases with the rotational speed of the shaker, but the final fibril mass is the same for all agitation speeds. To explain this contrasting behavior of aggregation kinetics, a population balance model is developed to account for the effect of shear on the aggregation of Aβ. The kinetic model includes primary nucleation, secondary nucleation, elongation, fragmentation, and depolymerization steps. The effect of steady uniform shear is encoded in the depolymerization rate constant (kd), and it is shown that kd decreases with shear rate initially and saturates at high shear rates. A competition between elongation and depolymerization rates yields different equilibrium masses of fibril at different shear rates. The model results agree quantitatively well with experimental data on the rate of aggregation and mass of fibrils as a function of shear rate. The modeling framework can be used to explain the shear rate-dependent aggregation of other amyloidogenic proteins.

We present a numerical study on a binary mixture of passive and circle swimming, self-propelling particles which interact via the Lennard-Jones (LJ) potential in two dimensions. Using Brownian Dynamics (BD) simulations, we present state diagrams using the control parameters such as attraction strength, angular velocity, self-propulsion velocity and composition. In a symmetric mixture, the system undergoes a transition from a mixed gel to a rotating passive cluster state and finally to a homogeneous fluid state as translational activity increases. The formation of the rotating cluster of passive particles surrounded by active and passive monomers is attributed to the combined effect of composition, activity and strength of attraction of the active particles. Different phases are characterized using radial distribution functions, bond order parameters, cluster fraction and probability distribution of local volume fractions. The present study addresses comprehensively the intricate role of activity, angular velocity, inter-particle interaction and compositional variation on the phase behavior. The predictions presented in the study can be experimentally realized in synthetic colloidal swimmers and motile bacterial suspensions.

Negatively charged nanoparticles are known to inhibit the fibrillation of amyloidogenic protein amyloid β (Aβ40), though the overall charge on the protein is negative. In this work a molecular dynamics study is reported to investigate the interaction of Aβ40 on negatively charged gold nanoparticles (3-5 nm) and charged (positive and negative) and neutral gold slabs. The equilibrium structures of Aβ40 on gold surfaces are characterized using residue-specific contacts on the gold surface, secondary structure analysis and binding free energy calculations. The simulation results reveal that the Aβ40 protein in water interconverts into β-sheets, which are building blocks of the mature fibrils, whereas on gold nanoparticles Aβ40 unfolds and adsorbs. Both the negatively charged gold nanoparticles and gold slabs arrest the formation of β-sheets in Aβ40, whereas the positively charged gold slab does not inhibit the formation of β-sheets. The residue-specific interactions between Aβ40 and the gold surfaces are important in governing the adsorption of Aβ40 on charged surfaces.