Interfacial slip plays a crucial role in a variety of fluid dynamics problems occurring in practical polymer processing, lubrication, adhesion, nanocomposites, etc. Despite many research efforts, a fundamental understanding of the underlying molecular mechanisms and dynamics is still lacking. Here, we present the intrinsic molecular characteristics of the slip phenomena by using atomistic nonequilibrium molecular dynamics simulations of polyethylene melts under shear flow. Our results identify three distinctive characteristic regimes with regard to the degree of slip and reveal the underlying molecular mechanisms for each regime: (i) the z-to-x chain rotation mechanism in the vorticity plane in the weak flow regime, which effectively diminishes the wall friction against chain movement along the flow direction, (ii) the repetitive chain detachment-attachment (out-of-plane wagging) and disentanglement mechanism in the intermediate regime, and (iii) irregular (chaotic) chain rotation and tumbling mechanisms in the strong flow regime. The second and third regimes can be classified as dynamically stable and unstable, respectively. The present findings would greatly help us comprehend the general characteristics of the interfacial rheological phenomena of polymeric materials.
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It is well known that liquid crystals (LCs) in dense solutions can exhibit tumbling (or kayaking), wagging, or chaotic motions in shear flow. While these dynamical characteristics of LCs under shear are apparently similar to those of polymer chains at interface in this study, there are several distinctive features of interfacial polymer dynamics in comparison with LCs. First, the wagging motion of dense LCs at intermediate shear rates is generally described in terms of the director (i.e., the average orientation vector of LCs). However, it has been recently reported that the individual LC molecules (mesogens) in the system still perform a rotational or tumbling motion during the wagging period [
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It is well known from existing simulation studies that the use of flexible walls where the wall atoms can oscillate around their equilibrium position would diminish the interfacial slip, as compared to rigid walls, because of an enhanced momentum transfer between polymer and wall (see, e.g., Ref. 15). In the case of flexible wall, the wall atoms (instead of fluid molecules) are typically linked to a thermostat and any heat generated inside the system is dissipated through the wall. Since the magnitude of the conductivities of fluids and wall should be finite, it is very likely to have a nonisothermal temperature profile to be developed within fluids and the profile would generally be dependent on the specific choice of the system thermostat [
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2017
The Society of Rheology
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