Thesis Title

Computer simulation of self-assembly of dipolar particles into gels or ordered structures to facilitate the design of "smart" materials

Co-Advisor: Orlin Velev

Systems of Interest

  • Dipolar colloid particles

Methods and Software

  • Discontinuous Molecular Dynamics (DMD) Simulations
  • Fortan 90
  • Rasmol molecular visualization program

Abstract

Colloid particles containing asymmetric or dipolar charge distribution self assemble into a variety of interesting microstructures such as nematic or smectic liquid crystals, co-crystals of novel symmetry (in mixtures of different size particles) and open networks (gels) containing long chains of particles. The multitude of possible structures that colloid particles can form makes experimental study of all possible variations infeasible. Alternatively, theory and computer simulation can be used to identify the structures of interest and to predict how the assembly process (kinetics, structure and thermodynamics) is affected by particle size and concentration, particle size ratio (for mixtures), electric field strength, dipolar interaction strength and location within the particle, temperature, and the nature, size and ionic strength of solvent and of solutes, so as to guide the discovery of advanced materials in the laboratory.

We have used discontinuous molecular dynamics to study the self assembly of dipolar colloid particles. Particles were modeled as hard spheres with two oppositely charged small spheres embedded in it. The charged small spheres were modeled as hard spheres with three step square shoulder repulsion between like charges and three step square well attraction between unlike charges. Dipolar particles formed three dimensional random network of cross linked chains at low packing fraction as shown in Figure 1.


Figure 1: Self-assembled structures (gel-like open network) formed at various temperatures at low packing fraction (temperature increases from left to right)

We calculated the orientation parameter and investigated the structures formed at various packing fraction and temperatures. We found that above certain packing fraction particles start arranging themselves into highly ordered structures as shown in Figure 2.


Figure 2: Self-assembled structures (highly ordered) formed at higher packing fractions

We will calculate gel transition point using percolation model so that we can find out minimum packing fraction (as a function of temperature) required to make gel from dipolar colloid particles. And then we will calculate the liquid-gel-crystal phase diagram (packing fraction vs. temperature) for systems containing dipolar colloid particles.

Publications

A. Goyal, C. K. Hall, O. D. Velev, "Bicontinuous gels formed by self-assembly of dipolar colloid particles," Soft Matter 6, 480-484 (2010).
A. Goyal, C. K. Hall, O. D. Velev, "Phase diagram for stimulus-responsive materials containing dipolar colloidal particles," Phys. Rev. E 77, 031401 (2008).
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Conferences

Thermo 2005 -- College Park, MD, April 28-30, 2005