A. The broad area of work being done in our group falls under the category,``Multiphase Reaction Engineering'', with a very strong emphasis on the reaction engineering aspects in systems containing micro-dispersed phases (such as emulsions, slurries, aerosols and even surfactant constituted micelles and microemulsions).

Some of the topics currently under focus are described below:

• Basic mass transfer with chemical reaction: The conventional theories of interphase mass transfer that have been around for decades and have been used extensively but these apply strictly to systems where the dispersed phase hold-ups are not very high. We are attempting to develop new frameworks within which mass transport and simultaneous chemical reaction can be analyzed for the scenario of high dispersed phase hold-ups. An example is that of a mildly stirred liquid-liquid reactor where the dispersed liquid phase occupies a large fractional volume and offers very high interfacial areas. The ``diffusion'' films around the dispersed droplets will now interfere with each other and conventional theories break down.
• Diffusion-reaction in emulsions: The use of organic solvents dispersed or emulsified in a reactive aqueous medium in order to enhance the transfer rates of a solute residing in a third phase (eg. a gas-liquid system being replaced by a gas-emulsion systems) is well known and often labelled as ``microphase catalysis''. However, the effects of the size of the emulsified droplets, solubilizing capacities and hold-ups, on the transfer rates is still not fully understood. We are interested in resolving these aspects and extending the microphase paradigm to situations where the emulsified droplets are large (compared to the diffusion film thickness) and their hold-ups are high. The currently available, pseudo-homogeneous models are not valid under these conditions and there is a need for rigorously heterogeneous approaches to this problem.
• Slurry reactors: The specific rates of reaction in gas-slurry systems are strongly influenced by the particle sizes and related characteristics of the slurry material. Our interest lies in examining these effects especially for the case when some of the particles are small enough to interact with the diffusing gaseous solute near the gas-slurry interface. Such an interaction is known to enhance the flux of the gaseous species being absorbed; however, particles are also likely to dissolve and this dissolution occurs near the interface as well as in the bulk slurry. These size changes, in turn, further affect the absorption rates. A theoretical framework for the scenario when the dissolution takes place exclusively in the interfacial zone, and its impact on the bulk slurry particle size distributions and the transient absorption rates, has been proposed. It will be attempted to extend this analysis when small and large particles are present together (as will happen in an industrial absorber) and obtain an experimental validation for these situations.
• Gas-liquid precipitators: Precipitation refers to the formation of solid particles in a medium in which these are sparingly soluble, in contrast to the more general phenomenon of crystallization where the supersaturation concentrations may be quite high. A variety of precipitated products are obtained by gas-liquid reactions, such as, precipitated calcium carbonate (PCC) which is made by carbonation of lime solutions or even slurries. The relationship between the operating parameters and the physico-chemical system properties on the product characteristics, such as, morphology, shape and precipitated particle size distributions is not understood very well in these multiphase systems. The product properties are important in that the end use to which these particles may be put depends upon these properties (eg. the kind of PCC obtained should have the ``right mouthfeel'' for use in toothpastes ie. size and texture are important). We are interested in exploring the entire map of these dependencies for industrially important systems so that the relation between the crystal nature and sizes, and processing/ reaction conditions is clearly established within an engineering science framework. Also important here are the use of various additives (eg. surfactants, alcohols, electrolytes) to influence the product characteristics by altering the solid-liquid wetting extents, solubilities of the chemical species etc.
• Slurry precipitators: When a gas-slurry reactive system leads to the formation of a solid product we have a situation in which reactant particle dissolution and product particle formation proceed simultaneously. These precipitators are thus a combination of the two previous items described above. Such systems are extremely relevant from the industrial viewpoint but very poorly understood on account of the complexities involved. We are interested in collating the information obtained from the dissolution and product formation studies (when only one of these is the dominant process) and developing an experimentally validated framework for the case when these occur together. One of the most important issues to be addressed is the interaction between the reactant and product particles that will co-exist, so that mixed clusters or flocs are likely to be formed. This interaction may also be expected to have a strong impact on the reaction rates and the product characteristics.
• NANOPARTICLES: The use of micelles and microemulsions to produce nanoparticles of inorganic salts (eg. silver chloride, cadmium sulfide) provides a novel route to the manufacture of these nanomaterials. The mathematical modeling of the formation of these particles, including the case of  complex/composite particles such  as core-shell particles, is an important area. Important issues are the role of intermicellar exchange processes and particle coagulation in the formation of the final product. Also, of interest is the formation of these particles in multiphase systems (eg. nanoparticles of calcium carbonate in gas-slurry-micellar systems). 
  

Our current focus is on:

• Coalescence behavior of emulsions in shear flows: With the objective of studying droplet coalescence in surfactant stabilized, oil-in-water emulsions, we have been examining the efficacy of simple shear flows in influencing the coalescence rates. Our preliminary investigations in which carefully prepared emulsions were sheared in a couette flow assembly, show that for a fixed surfactant concentration, the hold-up of the emulsified phase and the shear rates have a significant impact on the transient droplet size distributions. Some of the results that we have obtained are counter-intuitive - that greater shear can actually impart more stability to the emulsion. We are interested in unraveling the mechanisms that cause these effects and finding conditions under which these may be reversed. Much of this work is novel and there are many fundamental, challenging issues here, such as, the dimple formation due to inter-drop collisions, the rates of drainage of the continuous phase from these cavities and the consequent film rupture, the different probabilities of coalescence between droplets of similar and dissimilar sizes and so on. In functional terms our objective is to investigate the effect of shearing rates, fractional volumetric hold-ups of the dispersed phase, viscosity ratios of the dispersed/ continuous phases and the surfactant types and concentration, on the collision rates and efficiencies.

C. This is a new area in which we wish to initiate research. Our basic objective is to study the reaction engineering aspects of selected food product such as ``Food foams and sponges''.

Our initial focus will be on:

• Cell structure and foam formation in baked foods: The cell structure formation in baked foods such as breads and cakes arises on account of the bubbling of carbon dioxide (generated from fermentation or carbonating powders like baking soda) and water vapor through the mass of the dough loaded into the oven. These rise of the dough is accompanied by a variety of chemical reactions and marked changes in the physical properties of the expanding flour-fiber suspension. Since actual food products usually contain too many components, it will be attempted to organize baking experiments with standardized dough containing the barest possible additives. The evolving cell structure will be characterized in terms cell size distributions and these will be related to the processing conditions, chemical transformations and physical property changes via an engineering science approach. Food engineering is intrinsically very complicated due to the complex nature of the ingredients present and therefore studies tend to be purely empirical and recipe based. Our motivation is to inject engineering analysis into this area so that predictive and simulation capabilities, very important from the industrial, mass production point of view, are established. One of our objectives is also to develop this capability for specifically Indian food products.