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RESEARCH TOPICS

 

1.  Rheology of Complex Fluids

 

2.  Properties of Cross-linked Epoxy and its Nanocomposites

 

3.  Computational Studies of Cellulose degradation for Bioethanol

     Production

 

 

1.  RHEOLOGY OF COMPLEX FLUIDS

 

Rheological properties of complex fluids are directly related to their molecular structure.  We use a combination of molecular and mesoscale simulation techniques to decipher these structure-property relationships in complex fluids.  The focus areas of current interest are: 

 

(a) Effect of chain topology on the structural and dynamic properties of polymers in solution

 

We use molecular dynamics (MD) and multiparticle collision dynamics (MPCD) simulation methods to study the effect of chain topology on its structure and diffusion behavior in dilute solutions.  Simulation predictions are used to test theoretical predictions as well as explain experimental data for the effects of chain topology on its properties.  In recent work, we have applied this methodology to compare the behavior of cyclic and linear polymers in dilute solutions. 

 

(b) Determination of local viscoelastic properties by particle rheology simulations

 

In previous work, we used molecular dynamics simulations to investigate the cross-stream chain migration phenomena in nanochannels.  We have also shown that the molecular simulation results for the friction force on a particle translating in a fluid and the torque on particle rotating in fluid can be quantitatively explained by continuum mechanics expressions.  This principle is being extended to develop a new approach for determining the local viscoelastic properties of complex fluids by particle rheology simulations.  In analogy with the experimental particle microrheology technique, the viscoelastic properties of the medium are deduced in this approach by analysis of the probe particle motion in the medium. 

 

Relevant publications:

 

 

  1. Karim, M..; Kohale, S. C.; Indei, T.; Schieber, J. D.; Khare, R.; “Determination of viscoelastic properties by analysis of probe particle motion in molecular simulations”, Phys. Rev. E., 86, 051501 (2012).

  2. Hegde, G.; Chang, J.-F.; Chen, Y.-L.; Khare, R.; “Conformation and diffusion behavior of ring polymers in solution: A comparison between molecular dynamics, multiparticle collision dynamics and lattice Boltzmann simulations”, J. Chem. Phys., 135, 184901 (2011) . 

  3. Kohale S. C.; Khare R.; “Molecular dynamics simulation study of friction force and torque on a rough spherical particle”, J. Chem. Phys., 132, 234706 (2010).

  4. Kohale S. C.; Khare R.; “Cross stream chain migration in nanofluidic channels: Effects of chain length, channel height and chain concentration”, J. Chem. Phys., 130, 104904 (2009).

  5. Kohale, S.; Khare, R.; “Molecular simulation of cooperative hydrodynamic effects in motion of a periodic array of spheres between parallel walls”, J. Chem. Phys., 129, 164706 (2008).

  6. Khare, R.; Graham, M. D.; de Pablo, J. J.; "Cross-stream migration of flexible molecules in a nanochannel", Phys. Rev. Lett., 96, 224505 (2006) 

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2.  Properties of Cross-linked Epoxy and its Nanocomposites

 

The overall goal of this effort is to use the atomic-level resolution of molecular simulations to improve properties of cross-linked epoxy nanocomposites for end-use applications.  To this end, we have developed efficient methods for preparing atomistically detailed model structures of cross-linked epoxy and its nanocomposites.  The use of atomistic detail allows us to study the impact of specific chemical interactions on the properties of such materials.  We have previously used molecular simulations to study the dynamic heterogeneity in cross-linked epoxy in the vicinity of glass transition as well as the influence of the length of the cross-linker on the glass transition temperature of cross-linked epoxy.  For the nanocomposites, the effect of POSS chemistry on the properties of its nanocomposite with epoxy has been investigated.  The role of filler–matrix interfacial interactions and filler aggregation on the properties of epoxy – carbon nanotube nanocomposites is a topic of current interest. 

 

Relevant Publications:

 

  1. Khare, K. S.; Khare, R. “Effect of carbon nanotube dispersion on glass transition in cross-Linked epoxy – carbon nanotube nanocomposites: Role of interfacial interactions”, J. Phys. Chem. B, DOI: 10.1021/jp401614p, accepted (2013).

  2. Khare, K. S.;  Khare, R.; “Directed diffusion approach for preparing atomistic models of cross-linked epoxy for use in molecular simulations”, Macromol. Theory Simul., 21, 322 (2012). 

  3. Soni, N.; Lin, P.-H.; Khare, R.; “Effect of cross-linker length on the thermal and volumetric properties of cross-linked epoxy networks: A molecular simulation study”, Polymer, 53, 1015 (2012). 

  4. Lin, P.-H.; Khare, R.; “Glass transition and structural properties of glycidyloxypropyl-heptaphenyl polyhedral oligomeric silsesquioxane-epoxy nanocomposites: A molecular simulation study”, J. Therm. Anal. Calorim., 102, 461 (2010).  

  5. Lin, P.-H.; Khare, R.; “Local chain dynamics and dynamic heterogeneity in cross-linked epoxy in the vicinity of glass transition”, Macromolecules, 43, 6505 (2010). 

  6. Lin, P.-H.; Khare, R.; “Molecular simulation of cross-linked epoxy and epoxy-POSS nanocomposite”, Macromolecules, 42, 4319 (2009). 

 

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3.  Computational studies of cellulose degradation for      bioethanol production

 

Enzymatic hydrolysis of cellulose constitutes a crucial step in the production of ethanol from cellulosic biomass.  A detailed knowledge of the molecular mechanisms underlying the hydrolysis process is essential for developing strategies for enhancing the rate of ethanol production.  With this motivation, we use molecular simulations to study the interfacial thermodynamics in cellulose-water systems.  These simulations are performed in conjunction with Atomic Force Microscopy experiments carried out by our collaborators at Texas Tech.

 

 

 

Relevant Publications:

 

  1. Peri, S.; Muthukumar L.; Karim M. N.; Khare R.; “Dynamics of cello-oligosaccharides on a cellulose crystal surface”, Cellulose, 19, 1791-1806 (2012).

  2. Peri, S.; Karim M. N; Khare R.; “Potential of mean force for separation of the repeating units in cellulose and hemicellulose”, Carbohydr. Res., 346, 867-871 (2011). 

  

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For further assistance in these areas, contact Dr. Rajesh Khare.