Research

Research© 2012-16 Rohini Gupta

Post-doctoral research fellow under the guidance of Dr. Kathleen J. Stebe and Dr. Daeyeon Lee at the University of Pennsylvania, USA (2013-15)

Project: “Efficiency of conical fog collectors as inspired by Cacti”

  • Description: thermodynamic framework to describe directional motion of a drop along a conical structure, specifically in small Bond number regime, based on the equilibrium shape and position of an isotropic and axisymmetric liquid drop on a conical solid surface as a function of its volume.
  • Applications: design of efficient water harvesting systems.

Project: “Effect of structure and chemistry on wetting of cryogenic fluids”

  • Description: investigation of phase transition and wetting behavior of cryogenic fluids via condensation from a steady flux of gas at controlled pressure over surfaces with controlled temperature, using reflective interferometry; a LabVIEW interface to acquire time-lapsed images tagged with metadata from controllers and a MATLAB code for image processing and analysis.
  • Applications: production of specialty gases and performance of cooling systems.

Graduate Research Assistant under the guidance of Dr. Joelle Frechette at the Johns Hopkins University, USA (2007–13)
Dissertation:Investigation of Mechanisms Governing Electrowetting and Hydrodynamic Interactions in the Presence of Draining Channels” (ISBN: 9781303402616)

Project: “Role of hydrodynamic drainage forces in the mechanisms for tree frog adhesion and locomotion under flooded conditions”

  • Motivation: Central to the adhesion and locomotion of tree frogs are their structured toe pads, which consist of an array of 10 μm hexagonal epithelial cells separated by a network of 1 μm wide and 10 μm deep interconnected channels that end in mucus secreting glands. It has been proposed that the channels facilitate the drainage of excess fluid trapped between the toe pads and the contacting surface, and thus reduce the hydrodynamic repulsion during approach.
  • Description: The hydrodynamic interactions associated with drainage of a viscous fluid from the gap between a smooth and a structured surface (comprising of hexagonal array of cylindrical posts inspired by tree frog toe pads) are observed to agree with Reynolds’ lubrication theory at large surface separations, with deviations characterized by reduction in repulsion below a critical separation. Experiments agree with the scaling analysis, which allows estimation of a characteristic length scale (comprising of channel depth, width and periodicity) that corresponds to the transition from the fluid being radially squeezed out of the nominal contact area to being squeezed out through the network of interconnected channels.
  • Applications: design of biomimetic systems with reduced repulsion during approach and enhanced adhesion during retraction enabling robots to traverse flooded surfaces, designs of tire treads that minimize hydroplaning, and hydrofracture and micromodels for oil recovery.
  • Publications:
    Scaling Hydrodynamic Boundary Conditions of Microstructured Surfaces in the Thin Channel Limit
    Measurement and Scaling of Hydrodynamic Interactions in the Presence of Draining Channels
    Interferometry of Surfaces with Well-defined Topography in the Surface Force Apparatus

Project: “Electrowetting: electric field-induced changes in surface wetting and interactions at the solid−liquid interface”

  • Motivation: Recent needs for the design of low-power and efficient electrowetting systems have triggered studies to understand the mechanism and limitations of electrowetting.
  • Description: Our experimental results, consisting of height of a nanometer-sized annular water meniscus (formed via capillary condensation) measured as a function of the applied potential, unequivocally demonstrate that the local contact angle (or the solid−liquid interfacial energy) remains unaltered in electrowetting on dielectric and the mechanism at play is electromechanical in nature. Electrowetting response of substrates with contact angle hysteresis ranging from 1° to 30° is used to establish the relationship between contact angle hysteresis, threshold potential for liquid actuation, and electrowetting hysteresis. Our results demonstrate that the electrowetting hysteresis and the contact angle hysteresis are equal in magnitude and a modified electrowetting equation, based on balance of forces (including the pinning forces) acting on the triple contact line and on the drop, describes the electrowetting response of substrates with significant contact angle hysteresis.
    Potential-induced variation in contact angle hysteresis due to molecular reorganization at the solid−liquid interface (or change in the solid−liquid interfacial energy with applied potential) is used as a means to control macroscopic shape and motion of a drop on a homogeneous surface. Our results demonstrate that in situ and reversible modulation of contact angle hysteresis or pinning force with repeated switching between positive and negative potentials results in drop stretching and contraction, such that the motion of a drop subjected to a constant driving force (gravity) mimics the motion of an inchworm.
  • Applications: design and optimization of responsive surfaces as employed for digital microfluidics, variable focus liquid lens and electronic displays.
  • Publications:
    Invariance of the Solid-Liquid Interfacial Energy in Electrowetting Probed via Capillary Condensation
    Impact of Pinning of the Triple Contact Line on Electrowetting Performance
    Modulating Contact Angle Hysteresis to Direct Fluid Droplets along a Homogeneous Surface

B. Tech. Project under the guidance of Dr. Manish Vashishtha at the Malaviya National Institute of Technology Jaipur, INDIA (2006–07)
Thesis: “Modeling and Simulation of Formation and Growth of Holes in Thin Films”

  • Description: The focus of this project was to study the growth kinetics and morphology of holes in various stages of dewetting in thin apolar non-slipping Newtonian films subjected to long-range Lifshitz–van der Waals forces by the inclusion of Born repulsion based directly on numerical solutions of the thin-film equation.
  • Applications: floatation, coating and cleaning of surfaces, trickle bed reactors, and contact equipments for heat and mass transfer.

Young Engineering Fellow under the guidance of Dr. Santhanam Venugopal at the Indian Institute of Science, INDIA (Summer 2006)
Thesis: “Single-phase Synthesis of Thiol-derivatised Gold Nanoparticles using Modified Brust Method”

  • Description: The technique for synthesis of gold nanoparticles as reported by Brust being a two-phase synthesis indicates the occurrence of reaction at the water-toluene interface. The focus of this project was to investigate the reaction mechanism and study the variation of mean particle size with the rate of mixing of reactants. Our results confirmed that the reaction occurs in bulk, instead of the water-toluene interface.
  • Applications: contrast agents for cell imaging, therapeutics for treating cancer, detection of genetic sequence, drug delivery, tissue engineering, catalysis, and opto-electronic devices such as lasers, novel quantum computers, single electron transistors and memory devices.

Intern under the guidance of Dr. Dipayan Goswami at the Bhabha Atomic Research Centre, INDIA (Summer 2005)
Project: “Pilot Plant Study of Membrane Separation Technologies”

  • Description: The focus of this training was to analyze the performance of a 100 cubic meter per day capacity Sea Water Reverse Osmosis (SWRO) facility, which is the pilot plant for 1.4 mgd (million gallons per day) capacity Multi Stage Flash Distillation and Reverse Osmosis Hybrid Plant at Kalpakkam. It was established that ultrafiltration pre-treatment ensures high quality feed to a downstream reverse osmosis process, and consequently, superior performance of the same.
  • Applications: water purification techniques.
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