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Postdoctoral Research Work
A) Studying the Coarsening of Wet Foams in microgravity at International Space Station
Foams are dispersions of gas bubbles in liquids. They evolve rapidly on ground due to gravity-driven drainage. This work concerns experiments in microgravity that suppress drainage. In this way, wet foams are obtained, containing a substantial and controllable amount of liquid, while on ground one is usually limited to dry foams, with liquid fractions smaller than a few percent.
In many cases during generation, processing or final use, the volume fraction of gas is low at some point in the lifetime of the foams, making them “wet” (gas fraction below < 0.8). The study of such “wet” foams is particularly difficult on Earth because of gravitational drainage, which is very fast. On Earth foams evolve through three ageing mechanisms: gravitational drainage, coarsening and coalescence. Coarsening happens as gas transfers from smaller bubbles to larger ones because of pressure differences, and the rupture of the film between two bubbles causes coalescence. Both lead to an increase in the average bubble size in time. Out of the three mechanisms, drainage is the most understood, and as coalescence is a fast process it is currently actively studied on Earth. Coarsening is a slow process, and on Earth it is impossible to decouple it from drainage, which makes the controlled study of wet foams impossible on Earth. This is why the FOAM-C project carried out with European Space Agency focuses on the coarsening of wet foams, as they are studied on the International Space Station.
The theoretical frameworks to understand very dry foams and dilute collections of bubbles are firmly in place. When the foams are dry gas transfers through the thin films between the bubbles and the average radius evolves as , however when the volume fraction of bubbles is below close packing (around 0.64) there are no films and the bubble size evolves as . The question asked in our project is what happens at the intermediate liquid fractions, how do we go from gas transfer through the continuous phase and the power law of 1/3 to a film mediated growth and a power law of 1/2 as the gas fraction increases?
C) Investigating the settling of beads and rising of bubbles in viscoelastic fluid
The problem of particle settling and rising bubble experiments in fluid continues to be an active area of research in colloid science. Understanding of the settling velocity of solids and rising bubbles in fluids is of significant importance in many industrial applications, like drilling for oil and gas, packed and fluidized beds, geothermal drilling, mineral processing, etc. Rheological properties of the fluid play an important role on single settling particle or a single rising bubble and have been shown to change as the size of settling particle varies. Experiments on the settling beads and rise of single bubbles can tell us much about the boundary conditions during flow, which is important in foam ageing. We explore bubble rise and particle sedimentation in model fluids, where either the surface rheology or the bulk rheology are varied. Recent results from the group showed that understanding the rise of a collection of bubbles in viscoelastic fluids remains challenging to model. Therefore, we have simplified the system to start with single particles or gas bubbles and glass beads of different sizes, and study their motion in fluids. The fluids are viscous and viscoelastic solutions of a surfactant (CTAB) in the presence of sodium salicylate salt which concentration sets the bulk rheology.
We started with measuring the rise and settling of bubbles and glass beads in a viscoelastic fluid. Their velocities were measured in a surfactant solution of giant micelles, with well-characterised rheological properties. The rise of small bubbles (radius between 0.3 and 1.2 mm) of air containing vapor of a fluorocarbon (in order to slow down coarsening) and the fall of glass beads (radius between 1 and 3 mm) was measured. The viscosity of the solutions if 10 Pa.s at low shear rates (< 0.01s-1) and decreases to 10 mPa.s at large shear rates (1000 s-1). We see that depending on the particle size different settling velocity regimes are explored due to the shear thinning nature of the fluid.
B) Coalescence phenomena in Emulsion Stability using Diverse Range of Surfactants in O/W Emulsions
Coalescence is the main process of emulsion destabilization and understanding this mechanism in the presence of surfactants is critical to comprehend emulsion stability. The emulsion stability depends on many parameters, such as droplet size and water/oil ratio, and on the nature of the surfactants. Surfactants form a molecular thin film at the oil-water interface and provide surface viscoelasticity, which affects emulsion stability. Thus, we have been carrying out bottle test experiments using a model system of dodecane/water (w/o ratio 1:1), with a broad class of surfactants (cationic, nonionic, and copolymers). The emulsions were prepared using an Ultra-Turrax (IKA-Basic 18) and three levels of rotational speeds: 11000, 15000, and 24000 rpm. The water and oil heights were measured using a ruler with the value of division 0.5 mm
Ph.D. (Chemical Engineering)
Foaming Behavior of Aqueous Solutions of a Zwitterionic Surfactant in the Presence of Salts: Analysis of Specific Ion Effect and Synergism. Rheological and electrical properties were investigated for the oil/water emulsion system. The hydrated radii of the ions played a significant role in the foaming properties of the surfactant solutions while investigating the specific ion effect. The effects of salts on the foaming behavior of zwitterionic surfactant in an aqueous solution were investigated. The effectiveness of inorganic salts in reducing the foamability and foam stability was also examined. Role of oil in foam stabilizing mechanism was also analyzed. The surface tension and interfacial studies were conducted to study these effects. Specific ion effect was also investigated on the foaming nature of zwitterionic surfactant in the presence of 1:1 salts. The synergism between a zwitterionic and a cationic surfactant in presence of electrolyte was examined with respect to foamability and foam stability. Controlling and manipulating the foaming behavior of surfactant solution constitute the essence in numerous practical applications.
M. Tech (Chemical Engineering: Material Science and Technology)
Electrical Properties of Fluid-Fluid Interfaces in Presence of Anionic Surfactant and 1:1 Electrolyte: The Role of Adsorption and Surface Charge and its Importance in the Stability of Foams and Emulsions. Stability of hexane-in-water emulsions and aqueous foams prepared with aqueous solutions of an anionic surfactant and three 1:1 salt were examined. The effectiveness of these salts in defoaming and destabilizing the emulsions was investigated. This work showed that the ion-specific effect played a significant role in adsorption of ionic surfactants at fluid-fluid interface. From an application perspective, it played an important role in foaming efficiency of surfactant solution, controlling the droplet size in an emulsion and rate of destabilization of emulsions.
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