Reports: ND749866-ND7: Molecular Dynamics Simulations of Soft Lithography: Crossover between Molecular and Macroscopic Properties

Andrey V. Dobrynin , University of Connecticut

Adhesion phenomena play an important role in different areas of science and technology including tribology, colloidal science, materials science, biophysics and biochemistry. They are of paramount importance for nanofabrication and nanomolding, colloidal stabilization, drug delivery, interfacial friction and lubrication, cell mechanics and adhesion, and contact mechanics.

We have studied static and dynamic properties of the nanoparticle adhesion. In our approach we applied combination of the molecular dynamics simulations and theoretical calculations.  In the framework of our new model of the nanoparticle adhesion the deformation of the adsorbed nanoparticles is a function of the dimensionless parameter β=gp(GRp)-2/3 W-1/3, where G is the particle shear modulus, Rp is the initial particle radius, gp is the polymer interfacial energy and W is the particle work of adhesion. In the case of small values of the parameter β<0.1, which corresponds to strongly cross-linked large nanoparticles, the particle deformation can be described in the framework of the classical Johnson, Kendall, and Roberts (JKR) theory. Our simulations showed a significant deviation from the classical JKR theory in the case of the weakly cross-linked nanoparticles that experience large shape deformations upon particle adhesion. In this case the interfacial energy of the nanoparticle plays an important role controlling nanoparticle deformation. Our model of the nanoparticle adhesion is in a very good agreement with the simulation results and provides a new universal scaling relationship for nanoparticle deformation as a function of the system parameters.   

We have performed molecular dynamics simulations of pilling of nanoparticles to understand dynamics of the nanoparticle adhesion. In our simulations we have calculated the potential of the mean force characterizing the strength of the nanoparticle interaction with the substrate as a function of the particle substrate separation. These simulations have shown that the detachment of the nanoparticle from substrate occurs through  neck formation. The neck thickness decreases with increasing the nanoparticle shear modulus. Furthermore our simulations have established that the detachment time τR scales with the applied force as f -5. This strong dependence is a result of the fine interplay between nanoparticle deformation and its adhesion to the substrate that control nanoparticle shape.

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