Ethayaraja Mani

Stabilization of Pickering Emulsions with Oppositely Charged Latex Particles: Influence of Various Parameters and Particle Arrangement around Droplets.

In this study we explore the fundamental aspects of Pickering emulsions stabilized by oppositely charged particles. Using oppositely charged latex particles as a model system, Pickering emulsions with good long-term stability can be obtained without the need for any electrolyte. The effects of parameters like oil to water ratio, mixed particle composition, and pH on emulsion type and stability are explored and linked to the behavior of the aqueous particle dispersion prior to emulsification. The particle composition is found to affect the formation of emulsions, viz., stable emulsions were obtained close to a particle number ratio of 1:1, and no emulsion was formed with either positively or negatively charged particles alone. The emulsions in particle mixtures exhibited phase inversion from oil-in-water to water-in-oil beyond an oil volume fraction of 0.8. Morphological features of emulsion droplets in terms of particle arrangement on the droplets are discussed.

Sheet-like assemblies of spherical particles with point-symmetrical patches

We report a computational study on the spontaneous self-assembly of spherical particles into two-dimensional crystals. The experimental observation of such structures stabilized by spherical objects appeared paradoxical so far. We implement patchy interactions with the patches point-symmetrically (icosahedral and cubic) arranged on the surface of the particle. In these conditions, preference for self-assembly into sheet-like structures is observed. We explain our findings in terms of the inherent symmetry of the patches and the competition between binding energy and vibrational entropy. The simulation results explain why hollow spherical shells observed in some Keplerate-type polyoxometalates (POM) appear. Our results also provide an explanation for the experimentally observed layer-by-layer growth of apoferritin--a quasi-spherical protein.

Equilibrium and non-equilibrium cluster phases in colloids with competing interactions

The phase behavior of colloids that interact via competing interactions - short-range attraction and long-range repulsion - is studied by computer simulation. In particular, for a fixed strength and range of repulsion, the effect of the strength of an attractive interaction (ε) on the phase behavior is investigated at various colloid densities (ρ). A thermodynamically stable equilibrium colloidal cluster phase, consisting of compact crystalline clusters, is found below the fluid-solid coexistence line in the ε-ρ parameter space. The mean cluster size is found to linearly increase with the colloid density. At large ε and low densities, and at small ε and high densities, a non-equilibrium cluster phase, consisting of elongated Bernal spiral-like clusters, is observed. Although gelation can be induced either by increasing ε at constant density or vice versa, the gelation mechanism is different in either route. While in the ρ route gelation occurs via a glass transition of compact clusters, gelation in the ε route is characterized by percolation of elongated clusters. This study both provides the location of equilibrium and non-equilibrium cluster phases with respect to the fluid-solid coexistence, and reveals the dependencies of the gelation mechanism on the preparation route.

Effect of self-propulsion on equilibrium clustering

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.

Synthesis of single and multipatch particles by dip-coating method and self-assembly thereof.

We report a simple strategy to produce single and multipatch particles via the conventional dip-coating process. In this method, a close-packed monolayer of micron-sized silica particles is first formed at air-polymer solution interface, followed by dip coating of particles on a glass substrate. The simultaneous deposition of both polymer and particles on the substrate gives rise to a thin polymer layer and a monolayer of silica particles. Sonication of the substrate leads to the formation of a polymeric patch on one side of the particles. The patch shape depends on the aging of the polymer film prior to sonication. With aging time the patch evolves from ring-like to disk-like. This technique allows easy control of patch width by varying the concentration of polymer in the solution. We further show that the number of patches on the particle can be increased by controlling the concentration of silica particles at the interface such that surface coverage is less than that required for the formation of a close-packed monolayer. The single and multipatch particles are characterized by scanning electron and optical microscopy for the patch size, shape, and number distribution. The as-synthesized particles are used as a model to study self-assembly of colloids with electrostatic repulsion and patchy hydrophobic attractions due to polymeric patches. We find the formation of doublets and finite-sized clusters due to patchy interactions. Dip coating can be automated to produce large quantities of patchy particles, which is one of the major limitations of other methods of producing patchy particles.

Pickering emulsions stabilized by oppositely charged colloids: Stability and pattern formation.

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.

An analytical solution to the kinetics of growth of gold nanorods

We present the analytical solution of a mathematical model to explain the growth kinetics of gold nanorods grown via seed mediated synthesis. In the synthesis process, pre-formed gold nanoseeds are added to a solution containing cylindrical surfactant micelles, gold salt and a reducing agent. A mechanism is proposed based on several control experiments to understand the role of each species. Surfactant micelles act as a template for one-dimensional growth and solubilize the gold salt; the reducing agent partially reduces gold salt and the seed particles auto-catalytically reduce the partially reduced salt into metallic atoms. Based on this mechanism a mathematical model has been developed. Our model based on this physically relevant mechanism shows a bound exponential growth of nanorod length. The analytical model captures quantitatively the growth curves obtained in our experiments and the kinetic data of others under different synthesis conditions reported in the literature. The model can be appropriately modified to explain the growth kinetics of other seed-mediated growth processes of metallic nanorods.

SELF-ASSEMBLY DRIVEN CRYSTALLIZATION IN KEPLARATE-TYPE POLYOXOMETALATES

In this paper, we present direct evidences for two-stage mechanism of crystallization of patchy colloids from replica exchange Monte Carlo simulations. The patchy model colloid mimics the structure and interactions of a certain class of polyoxometalates (POM). We find that individual colloids self-assemble into two-dimensional sheets in hexagonal close-packed structure, and these sheets themselves stack to form crystals. The simulation explains the formation of hollow shell-like objects in POM solution [T. Liu, B. Imber, E. Diemann, G. Liu, K. Cokleski, H. Li, Z. Chen and A. Muller, J. Am. Chem. Soc.128, 15914 (2006)]. Simulation also predicted the formation of pentagonal caps that are essential for the formation of hollow, closed shells of POMs. Similar two-step crystallization of apoferritin protein was earlier found in experiments [S. T. Yau and P. G. Vekilov, Nature406, 494 (2000)]. The simulation study suggests nonclassical route to crystallization in patchy colloids.

Dual Role of Gold Nanorods: Inhibition and Dissolution of Aβ Fibrils Induced by Near IR Laser

Extracellular plaques of amyloid beta (Aβ) fibrils and neurofibrillary tangles are known to be associated with neurological diseases such as Alzheimers disease. Studies have shown that spherical nanoparticles inhibit the formation of Aβ fibrils by intercepting the nucleation and growth pathways of fibrillation. In this report, gold nanorods (AuNRs) are used to inhibit the formation of Aβ fibrils and the shape-dependent plasmonic properties of AuNRs are exploited to faciliate faster dissolution of mature Aβ fibrils. Negatively charged, lipid (DMPC) stabilized AuNRs inhibit the formation of fibrils due to selective binding to the positevly charged amyloidogenic sequence of Aβ protein. The kinetics of inhibition is characterized by thioflavin T (ThT) fluorescence, transmission electronic microscopy (TEM), atomic force microscopy (AFM), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). An increase in the aspect ratio of DMPC-AuNR in the range of 2.2-4.2 decreased the fibrils content proportionally. Further, the fibrils content is decreased by increasing the concentration of AuNR for all aspect ratios. As AuNR absorb near-infrared (NIR) light and creates a localized hotspot, NIR laser (800 nm) is applied for 2 min to facilitate the thermal dissolution of mature Aβ fibrils. Majority of Aβ fibrils are disintegrated into smaller fragments after exposure to NIR in the presence of AuNR. Thus, the DMPC-AuNRs exhibit a dual effect: inhibition of fibrillation and NIR laser facilitated dissolution of mature amyloid fibrils. This study essentially provides guidelines to design efficient nanoparticle-based therapeutics for neurodegenerative diseases.

Staggered Linear Assembly of Spherical-Cap Colloids

Linear assembly of colloidal particles is of fundamental interest in visualizing polymer dynamics and living organisms. We have developed a fluid-fluid interface-based method to synthesize spherical-cap polymeric latex particles. These particles are shown to spontaneously self-assemble in zigzag arrangement. The linear assembly is induced due to the shape anisotropy (one side is curved and the other side is nearly flat) and heterogeneous charge distribution on the particle surfaces. The necessities of these conditions are justified within the framework of DLVO theory. Spherical-cap particles of various size and aspect ratio reproduced the observed linear assembly, thus demonstrating the robustness of the self-assembly mechanism. While these types of assemblies are observed in spherical particles using microfluidic devices or electric field, the proposed approach is rather facile and does not require any external field. These novel assemblies could be potentially useful to understand kinetics of nucleation and growth of amyloidogenic proteins and to prepare artificial swimming microorganisms.