Smart Graphene Nanofluids

Graphene based nano-suspensions called Nanofluids have shown great potential to be the next generation of smart fluids. These nanofluids are prepared by dispersing properly functionalized Graphene in fluids such as water, ethylene glycol, oils etc. The suspensions show excellent thermal transport properties with ~10-100% increase in the thermal conductivity of the fluid and similar augmentation in forced convective heat transfer coefficients. They behave as ‘smart’ fluids and conduct more heat with increasing temperature. Graphene embedded collagen can lead to bio-nanofluids which is promising for use in cancer hyperthermia and targeted drug delivery.

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Living Polymers

Equilibrium/Living polymers are a class of molecules that spontaneously self assemble to form reversible aggregates of one or more type of molecules in a solution or in a melt. We study, through computer simulations, a model system of segments which can spontaneously assemble to form large polymeric aggregates. We show that strength of short range attraction and long range repulsion between segments leads to randomly networked gel states even in very dilute solutions. The short range attraction favors the growth of polymers and long range repulsion stabilizes the percolating network. It is shown that shear flow induces a non trivial structural transitions in these systems. They exhibit shear thinning behavior through lamellar/columnar structure formation at high shear stresses and through alignment of individual segments at low shear stresses. Such a decrease in the viscosity with shear rate due to shear alignment is reported in many experiments. (This work appeared as a cover page article in the journal Soft Matter).

This research highlight was submitted by Prof. P. B. Sunil Kumar.

Bandgap engineering by controlling microstrain and oxygen defects in BiFeO3 nanoparticles

BiFeO3 is a multifunctional material with numerous promising applications in multiferroic and photovoltaic devices. We show a simple way to tune the bandgap of BiFeO3 by controlling the microstrain and oxygen defects. The microstrain induced by oxygen defects reduces the rhombohedral distortion in BiFeO3 nanoparticles (< 30 nm) which is otherwise present in the bulk. This in turn gives a means for controlling the bandgap. Bandgap tuning in BiFeO3 enables it to be applicable for wide spectrum of applications. Figure: High-resolution TEM image of a BiFeO3 nanoparticle with Fast Fourier Transform images (FFT) shows differences in the local structure. The inverse FFT depicts the tensile and compressive strains (shown by white and black arrows) created in the lattice.

For more details click here. Complete article is available here.

Submitted by Dr. C. Sudakar.

Motion of drops on inclined surfaces in the inertial regime

When drops move on surfaces, a stress singularity arises at the contact line, due to which a classical analysis tells us that drops cannot move. To overcome this fallacy, various theories of what happens at the contact line have been proposed, most of which have been verified only at low velocities. Surprisingly, hardly anything was known about fast motion of drops on surfaces, which most of us would have seen on lotus leaves, in addition to its importance in designing self-cleaning surfaces and fast throughput microfluidics. We studied such fast motion of drops on surfaces for the first time and proposed theoretical expressions for the velocity of drops, based on the proposition that layers form inside the drops near the solid surface where the viscous effects are restrained. Inertia was shown to be important for the drop velocity, but not for the variation of the contact angle; a result which disproved many theories of contact line motion.

Submitted by Dr. A. P. Baburaj

More info here.

Unstable density driven transport across membranes.

Unstable density driven transport across membranes: Transport across thin porous surfaces, driven by unstable density difference across the surface, occurs in membrane separations, tissue transport, mantle convection, carbon sequestration etc. When a heavier layer of fluid is separated from a lighter layer of fluid below it by a horizontal microporous membrane, various regimes of transport occurs, based on the dominance of advection or diffusion. We discover a new regime of transport where advection balances diffusion, and remains so for about a decade of the dimensionless driving parameter, the Rayleigh number. The figure shows the top view of the pattern formed just above the membrane by the lighter fluid when it mixes with the heavier fluid in this new regime; the yellow lines are the lighter fluid and the green regions the heavier fluid. We propose a phenomenological explanation for the observed linear dependence of the dimensionless flux on the Rayleigh number in this regime.

Submitted by: Dr. A.P. Baburaj

Further details are here.

Modeling Biological Membranes

Modeling Biological Membranes morphological response to curvature inductors. Understanding stability of highly curved intracellular compartments , which are largely regulated by membrane associated curvature active proteins, have been a challenge. We have developed a Monte Carlo model to simulate fluid membranes with anisotropic directional curvature. This model is used to study membranes with an in-plane nematic orientational order and inclusions than can generate anisotropic curvature. Using this Monte-Carlo scheme we have shown that highly curved shapes, like tubes and discs, with a striking similarity to the structures engendered by certain curvature sensing peripheral membrane proteins, can be spontaneously generated by anisotropic directional curvature with nematic disclinations playing an important role. We then go on to show that, due to membrane mediated interactions these curvature inducing membrane nematogens can aggregate spontaneously, even at low concentrations."

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Posted by Prof. P. B. Sunil Kumar.

Bursting of gas bubbles

Bursting of gas bubbles at a free surface, a commonly occuring phenomena, is the source of fascinating,intriguing and unknown physics. The image sequence shows the main stages of collapse of a bubble of radius 2.15 mm. The event took 8 milli seconds to complete with the intervals between successive images being 0.25 milli sec, 0.75 ms, 0.75 ms, 1.25 ms and 3 ms. The bubble consists of a top thin film and a bottom cavity. After the thin film ruptures due to draining of fluid from the film, an unstable cavity is formed that collapses in a time period of milliseconds. The high energy focussing due to the collapse of millimeter sized bubbles results in jets with velocities of the order of 10m/s. Understanding the dynamics of this collapse and energy focussing could lead to, among many, new applications in printing, explosives, painless injections etc. This mechanism also determines to a large extent the rate of transfer of moisture to atmosphere from the ocean and the sound of rain.

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Posted by Dr. A. P. Baburaj.

Friction Stir Processing of Aluminium

Aluminium was subjected to friction stir processing (FSP) and the microtexture developed during the process was analysed by electron backscattered diffraction (EBSD). The color code corresponds to the orientation of the grains. The {111} pole figure (top images) and [001] inverse pole figure (bottom) show the orientations developed at the advancing, center and retreating side of the stir zone (i.e. either side from the center of the stir zone). The maximum intensity indicated is the times random orientation. It can be seen that there is a strong preference towards (001) orientation (represented by red colour) at the center.

Further details can be found at here and here.

Posted by Dr. Ranjit Bauri.

Process intensification using ultrasonics

Process intensification using high-intensity high-frequency acoustic fields (frequencies in the 20 kHz - 2 MHz regime) is an exciting field for research and industrial application. Acoustic enhancement of mass transfer, heat transfer, mixing, atomization, particle removal from surfaces, nano-particle synthesis by sono-fragmentation, chemical reactions, etc. is significant, scalable and cost-effective. Optimization for specific processes is under investigation in our state-of-the-art ultrasonic/ megasonic laboratory. "

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How far can you go?

Realizing limits of detection is an ongoing quest among scientists of various disciplines and this has implications to national security, safe drinking water, etc. Ammu Mathew and colleagues of the Department of Chemistry have shown that using unusual nanoparticles, called mesoflowers, sub-zeptomoles of trinitrotoluene (TNT), an explosive, can be detected selectively. The mesoflower emits red fluorescence due to the presence of atomically precise clusters attached on its surface. But in presence of tiny amounts of TNT, the luminescence turns to green. The sensor is so specific that even a closely similar molecule such as dinitrotoluene will not make the senor work. The limit of detection turned out to be just nine molecules. The same strategy has been used to detect hazardous mercury ions in water. This was reported in one of the top-most journals of chemistry, Angewandte Chemie.

Group leader: Prof. T. Pradeep

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