Deng Mingge, Li Xuejin, Liang Haojun, Caswell Bruce, Karniadakis George Em
Division of Applied Mathematics, Brown University, Providence, RI 02912, USA.
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, and Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, PR China.
J Fluid Mech. 2012 Nov 1;711. doi: 10.1017/jfm.2012.387.
Fabrication of functionalized surfaces using polymer brushes is a relatively simple process and parallels the presence of glycocalyx filaments coating the luminal surface of our vasculature. In this paper, we perform atomistic-like simulations based on dissipative particle dynamics (DPD) to study both polymer brushes and glycocalyx filaments subject to shear flow, and we apply mean-field theory to extract useful scaling arguments on their response. For polymer brushes, a weak shear flow has no effect on the brush density profile or its height, while the slip length is independent of the shear rate and is of the order of the brush mesh size as a result of screening by hydrodynamic interactions. However, for strong shear flow, the polymer brush is penetrated deeper and is deformed, with a corresponding decrease of the brush height and an increase of the slip length. The transition from the weak to the strong shear regime can be described by a simple 'blob' argument, leading to the scaling γ̇ ∝ σ, where γ̇ is the critical transition shear rate and σ is the grafting density. Furthermore, in the strong shear regime, we observe a cyclic dynamic motion of individual polymers, causing a reversal in the direction of surface flow. To study the glycocalyx layer, we first assume a homogeneous flow that ignores the discrete effects of blood cells, and we simulate microchannel flows at different flow rates. Surprisingly, we find that, at low Reynolds number, the slip length decreases with the mean flow velocity, unlike the behaviour of polymer brushes, for which the slip length remains constant under similar conditions. (The slip length and brush height are measured with respect to polymer mesh size and polymer contour length, respectively.) We also performed additional DPD simulations of blood flow in a tube with walls having a glycocalyx layer and with the deformable red blood cells modelled accurately at the spectrin level. In this case, a plasma cell-free layer is formed, with thickness more than three times the glycocalyx layer. We then find our scaling arguments based on the homogeneous flow assumption to be valid for this physiologically correct case as well. Taken together, our findings point to the opposing roles of conformational entropy and bending rigidity - dominant effects for the brush and glycocalyx, respectively - which, in turn, lead to different flow characteristics, despite the apparent similarity of the two systems.
使用聚合物刷制备功能化表面是一个相对简单的过程，这与覆盖在我们血管腔表面的糖萼细丝的存在情况相似。在本文中，我们基于耗散粒子动力学（DPD）进行类原子模拟，以研究聚合物刷和糖萼细丝在剪切流作用下的情况，并应用平均场理论来提取关于它们响应的有用标度关系。对于聚合物刷，弱剪切流对刷的密度分布或其高度没有影响，而滑移长度与剪切速率无关，并且由于流体动力学相互作用的屏蔽作用，其大小与刷的网孔尺寸相当。然而，对于强剪切流，聚合物刷会被更深地穿透并变形，导致刷的高度相应降低，滑移长度增加。从弱剪切状态到强剪切状态的转变可以用一个简单的“团块”观点来描述，得出标度关系γ̇ ∝ σ，其中γ̇ 是临界转变剪切速率，σ 是接枝密度。此外，在强剪切状态下，我们观察到单个聚合物的循环动态运动，导致表面流方向反转。为了研究糖萼层，我们首先假设一种忽略血细胞离散效应的均匀流动，并模拟不同流速下的微通道流动。令人惊讶的是，我们发现，在低雷诺数下，滑移长度随平均流速减小，这与聚合物刷的行为不同，在类似条件下聚合物刷的滑移长度保持不变。（滑移长度和刷的高度分别相对于聚合物网孔尺寸和聚合物轮廓长度进行测量。）我们还对具有糖萼层壁且在血影蛋白水平精确建模可变形红细胞的管内血流进行了额外的DPD模拟。在这种情况下，形成了一个无血浆细胞层，其厚度超过糖萼层的三倍。然后我们发现基于均匀流动假设得出的标度关系对于这种生理上正确的情况也是有效的。综上所述，我们的研究结果表明构象熵和弯曲刚度分别对刷和糖萼起主导作用，尽管这两个系统表面相似，但它们导致了不同的流动特性。
