Research Areas
We study how interfacial forces impact the dynamics of complex fluids such as suspensions, emulsions, foams, micro-emulsions, polymers and glasses. These are already crucial for industrial processes and products due to their peculiar rheology and flow characteristics. But we are now in an era where industries must maneuver through quick transitions in technologies under restricting resources, laying stress on novel materials that tailor to a purpose with a specific property.
Through a combination of experiments and theoretical models we create the desired characteristics by using fundamentals of solid mechanics, hydrodynamics, surface phenomena and statistical thermodynamics. We rely on experimental techniques of imaging, diffraction and scattering, rheometry and use computational techniques such as molecular dynamics and finite element methods.
Rheology of Polycrystalline Particles
Self-generating networks of organic crystals impart texture, strength, and stability to food, pharmaceutical, cosmetic products, and other materials such as crude oil. We study the in-situ crystallization and network formation at fluid-fluid interfaces or in the bulk of the liquid, quantify their rheological properties, and investigate the conditions under which they behave as “gels” or as “glasses.” Our objective is to tailor material properties to cater to process and industry requirements. We have developed technologies that enable:
1. Identification of the optimum composition and process combination to formulate stable Pickering emulsions for pharmaceutical and FMCG applications
2. Fracture of wax stabilized Pickering interface to break water in crude oil emulsions by novel surfactant combinations;
3. Breakage of networked suspensions by harnessing differences in electrical properties of two phases via application of strong electric fields; and
4. Growth of networks at interfaces (Pickering crystallization) and in bulk and the corresponding 2D and 3D rheological characterization of the solid-like behavior.
(a) Optical Micrograph of an emulsion with 1% w/w monoglyceride (MG) at the interface of PEG droplets in paraffin oil; Crystals are seen to encapsulate the droplets. (b) Artist’s rendering showing plate-like crystals forming interfacial network and spherulitic crystals forming bulk network.
Growth of wax crystals during cooling (a) without any additives, and (b) with 2000 ppm of silica nanoparticles grafted with octadecyl chains, (c) SEM images of wax crystals grown in the presence of grafted silica nanoparticles showing particles embedded in between wax layers, and (d) Elastic modulus of wax network during cooling. Modulus decreases upon addition of our nano-particles due to reduced network connectivity.
Engineering Polymers and Colloidal Particles to Control
and Tailor Crystal Habit
Particle shape has a bearing on physicochemical properties such as rate of dissolution, the perceived colour of pigments, packing density and flowability of powders, and rheological characteristics of wet and dry suspensions. Small molecules and salts dissolved in the mother liquor as growth-modifying additives have been long used to control the shape of particles formed during crystallization and their function is well understood. However, the growth-modifying action of larger molecules such as polymers and polymeric surfactants is relatively new.
We have demonstrated that similar hierarchical crystal habits can be achieved from the use of modifiers that span a range of lengths scales: molecules (< 2 nm), polymers (~5-10 nm), and colloidal particles (50-1000 nm). We have also correlated the rheological properties of the suspensions to the hierarchical crystal habits, quantified in terms of network connectivity and particle dimensions. The respective growth mechanisms for the various growth modifiers are also a focal point of our research.
Material Modeling & Simulation
Using molecular dynamics-based simulations we have been able to understand the underlying mechanism behind the association of similarly charged weak polyelectrolytes and the impact of additives on the kinetics of solute crystal growth from solutions to predict the influence of novel additives on the crystal habit of fatty acids.
The image represent effect of protonation, α, on the association of like-charged PEI chains estimated using all-atom MD simulations and the umbrella sampling technique. We show that only at certain protonation levels, self-assembly of PEI chains can occur, driven largely by the condensation of counterions (shown as purple spheres) between two chains. The potential of mean force (PMF) is seen to be the least for intermediate protonation (α=0.71) where larger fraction of counterions are condensed.
Industrial Manufacturing of Complex Phase Systems
Continuous manufacturing technologies with integrated control systems and IOT strategies are the key drivers for the so-called ‘Industry 4.0 revolution’. Implementing these approaches for the manufacture of complex phase materials is challenging as these systems are highly sensitive to the processing conditions and minor deviations lead to major changes in the microstructure, macroscopic rheology, and performance of these products.
Based on these design principles we have developed the following tools:
1. An end-to-end continuous technology with a modular approach that allows sequential addition of ingredients and inline unit operations with automated control for the manufacture of complex phase systems. Such machinery holds immense potential for the manufacture of emulsions, gels, soaps, and slurries.
2. A semi-continuous set-up for manufacturing of gelled yet flowable slurry fuels with high boron nanoparticle loading. The goal of the set-up is to produce highly energetic and stable slurry fuels for use in combustors.