Post Doctoral Work

Colloidal suspensions provide interesting opportunities for academic challenges as well as industrial applications. There are different ways to study these systems. I use rheology combined with optical microscopy to study the structure and dynamics of these systems. My focus at present is on these three areas:

1. Shear Thickening Rheology of Attractive Colloidal Systems: Dilute dispersions of fumed colloidal particles like carbon black and alumina show two different shear thickening transitions. The shear thickening observed at higher shear rates  is due to the breakdown of densified flocs and a concomitant increase in the effective volume fraction of the fractal particles in the fluid. This is in contrast to the well-known hydrocluster mechanism of shear thickening in concentrated hard-sphere and repulsive systems. At lower shear rates the increase in the apparent viscosity results from increased flow resistance due to the presence of gap-spanning log-like flocs in rolling flow. 

2. Aging and Internal Stress Dynamics in Colloidal Suspensions: Dilute suspensions of colloidal particles when subjected to shear rate quench, undergo gelation and show internal stresses which relax very slowly. We are trying to understand the role of these internal stresses in the aging dynamics of these systems.

3. Effect of Surfactant Additives on the Functioning of Engine Oil: This is an industrial project for Chevron to develop a laboratory method for predicting the effects of surfactant additives on the performance of engine oils.

PhD Work

I did my PhD with Prof. A. K. Sood in Department of Physics, Indian Institute of Science, Bangalore. My PhD thesis "Soft Matter Under Electric Field and Shear" is divided into two main themes:

1. Effects of electric field on colloidal suspensions: Motion of colloidal particles under the influence of applied electric field was observed under a microscope and were studied using image analysis and particle tracking.

(a) Frequency dependent shape changes of colloidal clusters under transverse electric field: Colloidal particles aggregate under the influence of an external ac field above a threshold. The frequency of the applied field decides the morphology of these aggregates. Below a critical frequency, the particles arrange themselves in 2-d triangular lattice. Above the critical frequency, the particles form long chains in a direction perpendicular to the applied field. The transition from the chains to triangular structure is reversible with frequency.

(b) Field enhanced recognition sensitivity between grafted ligands and receptors:  An ac electric field applied perpendicular to the confining walls increases the sensitivity of recognition of ligands by their corresponding receptors grafted on Brownian latex particles. Application of electric field assists the colloidal micro-particles grafted with receptors to come nearer due to electro-hydrodynamic drag. This increase in the local concentration of the latex particles results in improving the chances of ligand-receptor interaction leading to the aggregation of the latex particles. With this technique an increase in the sensitivity of the ligand-receptor recognition by a factor as large as 50 was achieved.

(c) 2-d systems formed by colloidal suspensions under AC electric field: Above a threshold field and below the critical frequency, colloidal particles aggregate into 2-d structures. A mono-dispersed colloidal suspension forms 2-d crystalline structure. However, aggregates formed by binary colloidal  suspensions are less ordered and can  be termed as  "2-d liquid". The 2-d structures thus formed are then used to study properties in 2-d systems. Mono-dispersed suspension were used to study freezing transitions in 2-d whereas the string-like cooperative motion seen in glass formers was observed in the case of binary suspensions.                                                         

2. Effects of shear on surfactant solutions: Linear and non-linear rheology of aqueous solutions of surfactants were studied using bulk rheology in a commercial rheometer. Aqueous solutions of surfactant systems were also studied using video particle tracking microrheology.

(a) One and two particle video particle tracking of microrheology of surfactant gels: One and two particle microrheology results were compared with the bulk rheology results from a commercial rheometer for aqueous solutions of CTAT. Two concentrations of CTAT were studied. One particle microrheology results were different from the bulk rheology results. Two particle microrheology results were similar to bulk rheology in case of CTAT 1.3% but were different in case of CTAT 2%.

(b) Non-monotonic rheological behavior of mixed surfactant system CTAB + SHNC: CTAB and SHNC in the molar ratio 2:1 form wormlike micelles which get entangled to form gels. Keeping the molar ratio same, the surfactant concentration was varied. Relaxation time and zero shear viscosity vary non-monotonically with surfactant concentration which can be  attributed to non-monotonic dependence of average micellar length on the surfactant concentration in the case of charged micelles.

(c) Rheology of an anionic surfactant system with a strongly binding counter-ion: The phase behavior and rheology of SDS+PTHC (sodium
dodecyl sulphate + p-toluidine hydrochloride) micellar solutions at different molar ratios (alpha=[PTHC]/[SDS]) of the two components show interesting features. At low values of alpha, polarizing microscopy observations reveal a transition from an isotropic to a nematic phase of disk-like micelles, whereas
a transition to a lamellar phase occurs at higher alpha values > 0.5, on increasing the surfactant content. Linear rheology of the isotropic micellar solution reveal a viscous behavior over a large range of surfactant concentrations. Surprisingly, this also extends to the nematic phase of disk-like micelles observed at alpha = 0.2 and surfactant concentration of 35%.

Instruments and Techniques Used

• Microscope : Nikon Optiphot2-Pol, Zeiss Axio Inverted Microscope.
  
• Image Processing and particle tracking: ImageJ, Matlab, IDL.
  
• Rheometer : Stress controlled Paar Physica MCR 300, MCR 301 and TA Instruments AR 1000N. Strain controlled ARES LS1.
  
• Dynamic Light Scattering : Brookhaven Instruments (Goniometer: BI-200SM, Correlator: BI-9000AT, Detector: Avalanche Photo Diode.)