Current Research
Here's a brief summary of research carried out in the Soft Matter Lab. Typically, the Publications page is most frequently updated.
Frequency-tunable electrohydrodynamics in oil-in-oil emulsions
Experimental model systems where hydrodynamic interactions can be controlled provide a playground for studying complex collective dynamic behaviours that emerge from the interplay of long-range electrostatic and hydrodynamic interactions. Both coherent (periodic) structures and incoherent (chaotic, turbulent?) structures are observed, and are switchable with the turn of a knob. We are currently studying the zoo of dynamical phenomena seen in this system with high-speed (white-light and fluorescence) imaging and time-resolved rheometry.
Student: Somayeh Khajehpour Tadavani. Collaborations: Shankar Ghosh, Shobo Bhattacharya, Atul Varshney (TIFR Mumbai).
Macromolecular Structure and Dynamics in Crowded and Confined Environments
Pulsed-field-gradient NMR spectroscopy yields ensemble-averaged local information about dynamics in diverse systems. A technical goal is to create model systems (colloids, vesicles...) that can be simultaneously studied by diffusion NMR and confocal microscopy. The former gives fantastic ensemble averages and works in opaque systems or systems where the building blocks are nanoscale. Confocal microscopy gives insights into single-particle motions. The scientific goal is understand mechanisms for anomalous diffusion of macromolecules in crowded and confined environments.
Structure and field-induced structure in thermo-sensitive microgel colloids
Ionic microgel colloids are fuzzy colloidal particles that are of interest for drug delivery applications because their structure (and fuzziness) is temperature sensitive. Recently we have found (collaborators: labs of Peter Schurtenberger and Priti Mohanty) that these ionic microgels are also sensitive to imposed electric fields. Yet, the internal structure of these microgels is far from fully characterized. We are simultaneously studying collective many-particle behaviours via confocal microscopy as well as probing single-particle structure via deuterium NMR (collaboration: lab of Mike Morrow).
Student: Suhad Sbeih. Collaborations: lab of Peter Schurtenberger (Lund), Priti Mohanty (Bhubaneswar), Mike Morrow (Memorial).
Lipid Vesicles and Colloid--Liquid Crystal Mixtures
Our goal is to expand study into colloidal building blocks that are less symmetric than microspheres. To this end we are trying to devise model monodisperse lipid vesicle system that we can study with microscopy, rheology and NMR.
One way to achieve greater control over defects in colloidal phases is to use anisotropic and spatially-structured fields. We are studying structure formation in colloid-liquid crystal composites.
Past Research Contributions
Colloids in Electric Fields: Tunable colloidal phase transitions
Soft matter is characterized by the importance of fluctuations and the ease with which one can control structure with external fields. We use the sensitive interplay of soft matter to external fields to control the interaction forces in colloidal systems. Static structure in three dimensions is obtained via fluorescence laser scanning confocal microscopy, and analyzed quantitatively using image processing software. We also study real-time dynamics and phase transition kinetics in micron-scale colloidal systems. The primary focus at the moment is electrohydrodynamics in colloids, and careful characterization of inter-particle interactions is turning out to be important
Students: Haruki Hirasawa, Tatsuo Izawa, Zena Aljabal. Collaborations: Stefan Egelhaaf & Juergen Horbach (Duesseldorf), Alfons van Blaaderen (Utrecht).
Patterned Materials: Pattern Engineering with and in Colloidal Crystals
We are interested in being able to make materials from colloidal templates with varying (and controllable & quantifiable) degrees of crystallinity.
Students: Payam Bagheri, Raheema Muhammad Aslam. Collaborations: Kristin Poduska (Memorial), Wenceslao González-Viñas (Navarra)
NMR studies of surfactant mesophases
Surfactant mesophases have complex memory effects arising from varying degrees of partial ordering making the data on solvent and surfactant diffusion in different phases (diffusion coefficients as a function of surfactant molar fraction) very noisy. We devised a method to look at well-aligned samples of non-ionic surfactant mesophases that allowed clean characterization of the phase diagram via diffusion coefficients, allowing us to make quantitative statements about the hexagonal and cubic phases. In another work, we tracked memory effects across phase transitions and thereby placed concrete limits on the structure of the lamellar phase.
The nematic-smectic phase transition in liquid crystals
This body of experimental and theoretical work on the nematic-smectic-A phase transition, including an invited review article (book chapter) demonstrated experimental studies that quantified mean-field order parameters and all prefactors for the first time via NMR spectroscopy and established the existence of a fluctuation-induced first-order phase transition via a new optical technique. The existence of a nematic-fluctuation-induced first order phase transition has been contested for decades, but was observed in our experiments. However, magnetic-field and concentration dependencies in our experiments suggested the need to include smectic fluctuations in the theory to account for discrepancies.
Soft Matter Book List
See here for a list of books that are relevant to soft matter and biophysics (thanks to Sabyasachi Dasgupta).
