Research Highlights on Soft Matter

Soft Condensed Matter Physics encompasses the study of liquid crystals, surfactants, neutral polymers, polyelectrolytes, colloids, emulsions, membranes and a large fraction of biomaterials etc. The realm of soft matter is very broad and currently presents fundamental scientific challenges which ultimately may produce deep economic and humanitarian impacts on the society. In "soft matter" the transitions among various phases are dominated by entropy. Ordered phases of soft matter can easily be distorted and therefore offer an ideal testing ground for fundamental concepts involving connection between symmetry, low energy excitations, and topological defects. Research in my group are carried out in the following areas with national and international collaborations and financial support from the NSF.


There is an increasing need to understand the self-assembling properties of short amphiphilic chain molecules as they find ample uses in forming templates in nano-fabrication of various devices. For example, semiconductor nano-structures are synthesized by the use of diblock copolymers as nano-lithographic masks. Broadly speaking, this is an emergent theme where it is believed that the self-assembling properties of amphiphiles and block copolymers can be utilized in parallel production of devices in nano-meso scales which are otherwise difficult to produce using conventional lithographic techniques. We have used Monte Carlo and Molecular dynamics simulations to explore the following:

  • Geometric effects in self-assembled structures
    • The effect of shape of an individual amphiphile has a marked effect on the self-assembled structures. But this aspect was not addressed in the previous work. We have addressed the
    effect of packing parameter by choosing hydrophilic elements of different sizes and derived important results. The figure shows the dependence of cluster distribution for different hydrophilic heads.


  • Phase diagram of complex molecules using Gibbs Ensemble Monte Carlo
    Quite naturally, we have extended the above calculations to find how packing constraints affect the phase diagram of amphiphiles with different hydrophilic heads. This is a natural generalization of applying Gibbs ensemble Monte Carlo method pioneered by Panagiotopolous and co-workers who applied this technique to polymers and amphiphiles on lattice. The accompanying figure shows how the critical point shifts in the temperature-density plane.


  • Statistical Mechanics of Lattice Amphiphiles
    Apart from carrying out Molecular dynamics and Monte Carlo calculations for models in continuum, we have also investigated amphiphiles on a lattice as shown in the figure. These calculations are order of magnitude faster, solvents molecules, which are eliminated in continuum calculations, here can be kept at no additional cost, and for certain properties lattice models are as good as their off-lattice counterparts. The figure on the left and right shows depiction of lattice amphiphiles and vesicle formation respectively.



    A translocating DNA through a 2-3 nano-meter protein channel on a membrane has attracted considerable attention due to substantial prospects of making synthetic nanopores for rapid detection of DNA and RNA sequences. Computer modeling using Monte Carlo and molecular dynamics simulation on coarse-grained models can bring out fundamental and universal aspects of the translocation processes. Our model incorporates Coulomb interaction as necessary. Channel current that is blocked by the translocating DNA and the hydrodynamics interactions are currently being incorporated to make the model more realistic. The figures show (left) a translocating hetero-polymer through a model pore and, (middle) histogram of passage time from simulation, and a



    Ionomers are polymers having a tiny fraction of charges at the backbone and equal numbers of counter-ions present in the vicinity for charge neutrality. Attractive Coulomb interaction among opposite ions pairs gives rise to formation of multiplets and bridges, as shown in the figure, responsible for gel like properties even at very low concentration. Ionomers also exhibit unusual shear thinning behavior and other anomalous viscoelastic properties. They are potential candidates for artificial muscles, fuel cell membranes, and self-healing materials to design exterior body of space-crafts. At a fundamental level, glass transition and formation of Coulomb gels in these systems is a nearly unexplored field. Unlike previous studies, we have developed modes for ionomers with explicit incorporation of ions and counter-ions and plan to make an in depth study of the structure-function relationship in these systems. The figure (right) shows a snapshot of formation of multiples from our simulation.

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    The polymer-induced depletion interaction between mesoscopic colloid particles in a solution of non-adsorbing polymer chains is of fundamental interest in colloid physics. For entropic reasons the chains avoid the space between two close particles, or between a particle and a planar wall, and create an effective attraction among the colloid particles, or push the particles toward the walls of the container. This depletion interaction has been used to explain phase diagrams of colloid-polymer mixtures and is believed to be important for a variety of interesting colloid systems such as casein micelles, hemoglubine, and globular proteins. We have used an off-lattice bead-spring model of a polymer solution in a container with impenetrable walls to study the depletion interaction of a colloid particle with the planar wall by means of a Monte Carlo simulation. As expected, this interaction is found to depend essentially on the ratio R/Rg of the particle radius R to the mean radius of gyration Rg of the polymer chains in the case of dilute and semi-dilute solutions.



    The driven diffusion of an entangled polymer in disordered media is a theoretical problem that has received much attention because of its close connection to Biology. The enormous motivation stems from the prospect of high speed detection of sequences in a single polynucleotide molecule. As a result, the behavior of large flexible molecules e.g., DNA, RNA, proteins, and synthetic polyelectrolytes in disordered porous media offers many challenges and excitement to the scientific community. Therefore, it is rather important to identify different mechanism of transportation of polymers as it moves through a disordered medium where the chain entanglement effect plays a crucial role.\par We have examined the properties of an end-labeled telechelic polymer chain where only the first monomer of the chain is influenced by an external force using a stochastic molecular dynamics simulation method. and calculated the dynamic properties of the chain directly from the simulation as a function of bias and impurity density as well.

    Conformations of a telechelic chain of length N=64 in porous media with rho_imp= 0.1. In both cases the external field points from left to the right. The top snapshot shows a situation where the rest of the chain has overtaken the immobile head which has got stuck between obstacles. The left snapshot displays a freely drifting chain among obstacles. Most of the obstacles have been removed for better visibility.




    Systems quenched from a high temperature disordered state to a low temperature ordered state do not order instantly. Different broken symmetric states come into competition to select the final equilibrium state. Coarsening or phase ordering dynamics is the study of evolution of the system towards the final equilibrium configuration. At late time typically a single length scale R(t), which is the average distance between the defects governs the late time phase ordering dynamics and other microscopic details become irrelevant. The system enters into a scaling regime. The domain size R(t) ~ t 1/z, and the equal time pair correlation function C(r,t) takes a simple scaling form f(r/R(t)).

    A rather different picture emerges when the phase separating system is confined in restricted geometries. We have studied the phase ordering dynamics of a binary liquid mixture and nematic liquid crystals in restricted geometries under different surface anchoring conditions. We find that depending upon the symmetry and the conservation laws of the order parameter, geometric confinement and surface anchoring have marked impact on the growth dynamics.

    We have extended these calculations from simple mixture to a phase separating polymer solution where the coarsening process gets more complicated by chain conformations and entanglement effects. Our simulations strongly indicate that the true late time growth kinetics of quenched polymer solutions belong to the same universality class of small molecular mixtures.

    Following the general theme, the above study has been extended to the case where polymers are confined in a long and narrow cylindrical pore subject to an attractive wall potential. We find that for deep quenches the uniform density state breaks up into alternate polymer rich phase in the form of plugs and the solvent rich phase.