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Nanoparticle translocation through polymer brush decorated nanochannels: General principles

Applicant Dr. Holger Merlitz
Subject Area Experimental and Theoretical Physics of Polymers
Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Term from 2020 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 449432865
 
Final Report Year 2024

Final Report Abstract

We have developed a mean-field theory which describes polymer brushes on cylindrical substrates (both on the outside of a cylinder and on the inner wall of a nanochannel). Since the surface tension is a function of the brush thickness, brushes in nanochannels can show a steep collapse transition upon a modification of solvent quality. This is in contrast to brushes on planar substrates, which always exhibit a continuous collapse transition when the solvent quality drops. We have shown that this transition turns continuous in nanochannels as well if the contributions of chain elasticity are sufficiently large, as is the case at high grafting densities. The role of elasticity in the context of the collapse transition inside nanotubes has not been investigated before, and our project has led to an improved understanding of the mechanisms behind the reversible gating features of brush decorated nanochannels. If the brush inside a nanochannel is sufficiently swollen to occupy the entire volume, then the channel is in its closed state. Using external pressure, nanoparticles can be pushed into and translocate through the closed nanopore. There exists a certain parameter range in which multiple nanoparticles form droplets (or clusters) that are driven through the channel. The droplet size is a function of the pressure the nanoparticles are exposed to. The frequency of droplets is proportional to the flux of incoming nanoparticles. If critical values of influx and/or pressure are reached, the flow of separate droplet transforms into a continuous flow through a permanently open channel. We have investigated the corresponding dynamic phase transition and set up the phase diagram. The quantitative understanding of this dynamic phase transition is of interest for the design of nanofluidic rectification devices. We have further compared the effects of various thermostats used in the MD simulations on the properties of the nanoparticle flow inside brush decorated nanochannels. Since this is a driven non-equilibrium system, the DPD thermostat is generally regarded to be superior to the common Langevin thermostat (LGV). But DPD simulations are computationally more demanding and generate, for our example, about 4× the computational costs than the LGV thermostat. In the literature, there are suggestions to use a modification of the LGV thermostat in which the thermostating is disabled in the direction of the driving vector. We have shown that this LGV-thermostat yields results that are incompatible with both the DPD and the conventional LGV thermostat, so that it is not suitable to be used for the current system. Contrary, the DPD and the LGV yield very similar phase diagrams and also compatible scaling properties of some of the droplet parameters, so that the LGV thermostat could indeed be applied here to generate preliminary results at reduced computational costs.

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