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Mechanism of Nonmonotonic Increase in Polymer Size: Comparison between Linear and Ring Chains at High Shear Rates
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posted on 2019-10-18, 17:45 authored by Zhenhua Wang, Qilong Zhai, Wei Chen, Xiaoliang Wang, Yuyuan Lu, Lijia AnThe static and dynamic behaviors
of linear and ring polymers under
shear flow over a wide range of shear rates are studied using a hybrid
simulation method that couples multiple-particle collision dynamics
with molecular dynamics. The results reveal that the polymer size
increases monotonically with increasing shear rate when hydrodynamic
interactions are ignored, in agreement with classic theoretical predictions.
However, for the cases with hydrodynamic interactions, due to the
transition from a linear to a nonlinear velocity profile, the size
of both linear and ring chains exhibits a nonmonotonic dependence
on the shear rate, and this counterintuitive behavior could be divided
into three main regimes. At specific shear rates, linear polymers
exhibit a relatively stable stretched state and a rapidly rotating
collapsed state, which correspond to the maximum and minimum sizes,
respectively. Although the rings behave similarly to the linear polymers,
there exist two different relatively stable stretched states: one
with an oval-shaped conformation and the other with an S-shaped conformation.
For the oval-shaped state, the tumbling motion almost disappears but
the tank-treading motion persists, whereas for the S-shaped state,
both tumbling and tank-treading motions are greatly suppressed. Moreover,
contrary to previous theoretical predictions, a noticeable bulge is
observed for the polymer size in the gradient direction and the alignment
angle deviates considerably from theoretical prediction, indicating
the existence of relatively stable collapsed conformations at large
shear rates for both linear and ring polymers. However, in ideal linear
shear flow introduced by using the “fix deform” command
to Dissipative Particle Dynamics system in LAMMPS package, both linear
and ring polymers exhibit a monotonic dependence of size on shear
rate. These results shed new light on the understanding of the dynamic
response of linear and ring polymers in ultrahigh shear flows.