Mesoscale simulations of polymer dynamics in microchannel flows

The non-equilibrium structural and dynamical properties of flexible polymers confined in a square microchannel and exposed to a Poiseuille flow are investigated by mesoscale simulations. The chain length and the flow strength are systematically varied. Two transport regimes are identified, corresponding to weak and strong confinement. For strong confinement, the transport properties are independent of polymer length. The analysis of the long-time tumbling dynamics of short polymers yields non-periodic motion with a sublinear dependence on the flow strength. We find distinct differences for conformational as well as dynamical properties from results obtained for simple shear flow. Introduction. – Confinement fundamentally alters the properties of dilute polymer solutions – compared to bulk behavior – when either the polymer radius of gyration is on the order of the characteristic dimensions of its proximity [1–4], and/or an external field is applied, e.g., a shear or pressure field. In the first case, geometrical constraints lead to a stretching of the polymer parallel to the surfaces [1–6]. In the second case, in addition to flowinduced deformations, polymer-surface hydrodynamic interactions determine the polymer dynamics and leads to, e.g., cross-stream migration [7–12]. The migration effect has been studied intensively for DNA-like molecules by computer simulations and the relevance of hydrodynamic interactions has been confirmed. Experimental studies of individual DNA molecules in steady shear flow by fluorescence microscopy have provided a wealth of information on single polymer dynamics [13–16]. In particular, large conformational changes have been revealed due to tumbling motion [15, 17, 18]. Similar studies for flexible polymers confined between surfaces or in narrow channels have not been conducted so far, however, a similar complex dynamics can be expected. Understanding of single polymer behavior is of paramount importance for the emerging technology of microfluidic devices. Insight into the detailed microscopic conformational, dynamical, and transport properties of polymers, e.g., DNA, will help in the conception and design of such devices. Moreover, such studies will contribute to the understanding of the transport properties of biological macromolecules through blood vessels. The proper account of hydrodynamic interactions is essential in simulation studies of fluid flows in channels as is emphasized by the appearance of cross-streamline migration. Recently developed mesoscale simulation techniques, such as Lattice Boltzmann simulations [12, 19], Brownian dynamics simulations with a hydrodynamic tensor [4], and multi-particle-collision dynamics [20, 21] (also called stochastic rotation dynamics), are well suited for simulations of microchannels flows and are able to bridge the lengthand time-scale gap among the solvent and solute degrees of freedom thereby taking hydrodynamic interactions adequately into account. In this letter, we will present results for the conformational and dynamical properties of polymers confined in a square channel and exposed to a Poiseuille flow by mesoscale computer simulations. Both, the polymer length as well as the pressure gradient are systematically varied. The considered polymers fall into the crossover regime from weak to strong confinement. Experiments [11] and simulations [8] predict a distinct different flow behavior in the two limits, which is confirmed by our simulations. The strong confinement regime has received less attention so far, it is however most relevant if microchannels are intended to be used for DNA characterization. In addition, we discuss chain orientation and tumbling dynamics, aspects which have not been addressed in previous studies. Simulation method, model. – We use a hybrid simulation approach to study the properties of flexible polymers in flow, where molecular dynamics simulations