Guiding atomic and molecular self-assembly by structured surfaces is a well-established route for fabricating functional surface nano and micro structures. Surface guided self-assembly of micron-scale particles (colloids) can not only shed light on the various processes at play, often inaccessible at the atomic and molecular length scales, but also help realize materials for applications that include structural color filters and photonic bandgap devices.
Research in this area in our group focuses on extending atomic epitaxial growth concepts to microparticles. In due process, we not only attempt to identify phenomena unique to colloidal surface growth but also develop new design principles in overcoming the experimental challenges in extending atomic surface growth ideas to the colloidal realm.
When you cool a liquid rapidly enough to avoid crystallization, particle dynamics slows down and eventually ceases at the glass transition. Unlike conventional equilibrium critical phenomena where a diverging relaxation time is due to growing static correlations, no obvious structural changes accompany the glass transition. The grand challenge in contemporary condensed matter physics is to ascertain if the slowing down of dynamics en route to forming glass is a purely kinetic phenomenon - structure has no role - or a thermodynamic one - has a structural origin.
Research in this area in our group aims to address the following issues:
Is there a structural origin for the slowing down of dynamics on approaching the glassy state?
Which of the current theoretical scenarios capture best the observed slowing down?
How do glasses flow?
With recent advances in particle synthesis, it is now possible to tailor colloidal building blocks that rival the complexity in interactions and shape found in atomic and molecular ones. The picture shows colloidal particles synthesized in our group following established protocols.
Our goal is to understand the role of anisotropy in particle shape and interactions on the dynamics and phase behaviour of suspensions.
"Crystals are like people, it is the defects in them which tend to make them interesting" - Colin Humphreys
The role defects play in determining the properties of crystalline materials cannot be overstated. They are not only vital in determining the yield strength and the deformation mechanisms of crystals but also dictate their electronic and optical properties. Elucidating the dynamics of defects with and without external perturbations continues to remain a key goal of materials research. The difficulty with atomic systems, however, is that the length and time scales are too small to capture the dynamics of defects and interactions between them.
By using micron-scale colloidal particles as substitutes for individual atoms, we scale up the problem so that defect dynamics can be observed in the presence and absence of external mechanical forcing and in real-time using a confocal-rheoscope.
Micro- and nanoscale motors and engines operate in a regime swamped by fluctuations. Unlike the familiar macroscopic engines, where the large number of degrees of freedom (DOF), ~ O(10^23), of the working fluid, essentially smothers away fluctuations, for microscopic ones, the number of DOF ~ O(10^1) and fluctuations in key engine parameters like efficiency and power become apparent. Thus, albeit infrequently, a biological motor carrying cargo within a cell can stall and even take a backward step and a micrometer-sized colloidal heat engine can operate in reverse as a refrigerator.
Theoretical advances in stochastic thermodynamics have helped elucidate the mechanisms of energy transduction of these micro- and nano-sized machines operating between equilibrium thermal baths. In many situations, however, the bath themselves can be out-of-equilibrium, inside a living cell for instance. Our goal is to understand the influence of the non-equilibrium nature of the bath on the performance of these engines.
Unlike Newtonian liquids wherein the viscosity remains constant over a wide range of imposed shear rates, in soft materials the strong coupling of the underlying mesoscopic structure to the imposed shear often results in remarkable and non-trival dependence of viscosity on shear rate. While the phenomenon of shear-thinning - wherein the viscosity reduces with shear rate - is ubiquitous in soft materials, the opposite trend - shear-thickening - is mostly seen in surfactant systems, and dense colloidal and granular suspensions (check out this cool video..). Current mechanisms that attempt to explain shear-thickening include, but are not limited to, the formation of hydroclusters and the interlocking of particles due to friction.
By combining rheological measurements with simultaneous 3-dimensional single-particle imaging using a confocal microscope, we are currently exploring the role of particle shape on the rheological behaviour of colloidal suspensions.