RAJAGOPALAN, Raj

Professor

PhD, MSc (Chem. Eng.) Syracuse University, NY 1975
BTech (Chem. Eng.) IIT, Madras, 1969

Contact information
Blk EA, 4 Engineering Drive 4, #07-39, Singapore 117576
Tel: (65) 6516 4679   Fax: (65) 6779 1936
Email: cherajr@nus.edu.sg

RESEARCH

Physics of Polymer Interfaces: Going Beyond Mean-Field Approximations

This theoretical program addresses the adsorption of polymers on surfaces and the resulting structure of the layer. We have developed a new computational technique (known as the Contact Distribution Method, or CDM) based on statistical thermodynamics to determine forces of interaction in polymer/colloid systems. As this method is exact, it is now possible to examine the severity of mean-field assumptions (typically made in self-consistent field theories of polymer layers, solutions, and blends) so that improved theories can be constructed. Notice that the mean-field approximation severely limits the usefulness of the theories to practical situations. Examples include “starved” surfaces and adsorption on heterogeneous surfaces, which can be addressed using our method. Also important are random copolymers, associating polymers, and chains of complicated molecular architecture (e.g., polymers with side chains), which are difficult to deal with within the mean-field framework. The CDM also allows one to deal with curved surfaces and particles of the order of the radii of gyration of the polymers. The scope of this research program is not limited to polymer layers and interfaces but includes single chains as well. We plan to use these results to improve self-consistent-field approximations and the renormalized van der Waals free energy functional methods (as the latter can provide analytical expressions). Extensions of the self-consistent mean-field theory to curved surfaces, fluid/fluid interfaces, and short-chain surfactants are also planned.

Single-Particle and Single-Chain Force Spectroscopy: Harnessing the Power of Light

This experimental program is designed to complement the above theoretical program and takes advantage of the recent developments in optical trapping of particles. The basic approach combines scanning near-field optical microscopy (SNOM) with diffraction-limited optical traps (known as tweezer traps) to build an optical force microscopy (OFM). In the OFM, an optically held probe particle serves as the scanning probe (e.g., like the tip of an atomic force microscope, but without the mechanical arm) so that soft interfaces (such as a polymer layer or a surfactant-laden oil/water interface) can be imaged and probed. Here, the optical traps serve as force transducers. As the optical radiation and gradient forces are quite sensitive to laser power and, further, can be controlled dynamically, this instrument can therefore be used to measure very small (sub-picoNewton-level) forces and to manipulate polymer layers and single polymer chains.

Brownian Fluctuation Spectroscopy: Harnessing the Power of Thermal Noise

This program attempts to harness the power of the ever-present thermal noise in a system to extract micromechanical and microrheological information on soft interfaces and bulk complex fluids. We call this technique Brownian Fluctuation Spectroscopy (BFS), as the thermal noise of the system manifests itself through the Brownian fluctuations of the constituents of the system (e.g., polymer chains) or through the Brownian fluctuations of probe particles, and since such fluctuations are examined through spectral densities. As described in the extended descriptions, the use of Brownian fluctuations allows one to use standard instrumentation when special instrumentation is either impractical or economically prohibitive. Therefore, a particular advantage of the technique is that dynamic light scattering or particle tracking techniques can be used. Further, adaptation of the technique to online instrumentation can also be easily achieved. Our focus in this area consists of (i) development of the basic concepts and the needed theoretical framework, (ii) establishment of the relation between microrheology and macrorheology, (iii) use of atomic force microscopy in combination with BFS to study polymer interfaces, (iv) use of standard dynamic light scattering and diffusing wave spectroscopy to examine the microrheology of bulk systems, and (v) adaptation of the optical trapping technique described above for BFS.

Stability of Supercooled Soft Systems: Resolving the Competition between Thermodynamics and Kinetics

Nucleation/crystallization phenomena determine the performance of many processing operations in materials and biological sciences (e.g., the growth of large, defect-free single crystals for structural analyses and the development of novel nanocrystalline materials and composites). In the case of “soft” systems, issues such as crystal structure and stability and the role of interaction forces on the dynamics take on new significance because of the possibility of using self-assembly of biological, macromolecular and colloidal dispersions for a number of advanced materials (e.g., photonic bandgap materials). Even in the case of atomic systems, a number of problems on the atomic scale structure and dynamics of crystal growth (e.g., the interplay between thermodynamics and kinetics on the interfacial structure and dynamics and stability of embryos and small crystallites) remain unaddressed. In fact, answers to even basic questions such as to what extent the classical equilibrium or quasi-equilibrium approximations made in theories of crystal growth are justified remain elusive. A major reason for the dearth of information in this area is the difficulty of studying atomic systems at the microscopic scale due to the very small length and time scales of the relevant phenomena. In addition, one faces physical and experimental constraints in varying the interaction force parameters arbitrarily.

However, charged colloids provide a physical model of supercooled liquids that is sufficiently simple and yet rich in the essential phenomena and, thus, open a hitherto unavailable window to the microscale thermodynamics and kinetics of crystal growth. This program takes advantage of this unique opportunity to gather information of significant research and pedagogical impact on: (i) how the details of interaction forces affect the nucleation and growth of crystalline materials, and (ii) the structure and dynamics of the crystal/melt interface and phase transitions. Also of interest is the influence of the degree of metastability on the kinetics of structural transitions.

SELECTED PUBLICATIONS

Sohn, I-S. and Rajagopalan, R., “Microrheology of model quasi-hard-sphere dispersions”, J. Rheology, 48, 117-142 (2004).

Sohn, I-S., Rajagopalan, R., and Dogariu, A. C., “Spatially resolved microrheology through liquid/liquid interfaces”, J. Colloid Interface Sci., 269, 503-513 (2004).

Rangarajan, M., Jimenez, J., and Rajagopalan, R., “Effects of polymer layer anisotropy on the interaction between adsorbed layers”, Macromolecules, 35, 6020-6031 (2002).

Popescu, G., Dogariu, A. C., Rajagopalan, R., “Spatially resolved microrheology using localized coherence volumes”, Phys. Rev. E, 65, 041504 (2002).

Rajagopalan, R., “Simulations of self-assembling systems”, Curr. Opin. Colloid Interface Sci., 6, 357-365 (2001).

Jimenez, J., de Joannis, J., Bitsanis, I., and Rajagopalan, R., “Bridging of an isolated polymer chain”, Macromolecules, 33, 7157-7164 (2000).

de Joannis, J., Jimenez, J., Rajagopalan, R., and Bitsanis, I., “A polymer chain trapped between athermal walls: Concentration profile and confinement force”, Europhys. Lett., 51, 41-47 (2000).

Dogariu A. C., Rajagopalan, R., “Optical traps as force transducers: The effects of focusing the trapping beam through a dielectric interface”, Langmuir,16, 2770-2778 (2000).

Chen,S.H.,and Rajagopalan,R., Editors, Micellar Solutions and Microemulsions: Structure, Dynamics, and Statistical Thermodynamics, Springer., Verlay, NY (1995).

Hiemenz, P. C., and Rajagopalan, R., Principles of Colloid and Surface Chemistry, 3rd Edition, Taylor & Francis, NY (1997).