Effect of Surface Nanopatterning on Slip: The Case of Couette Flow of Long-Chain Polyethylene Melt Flowing Past Gold Surfaces

The manifestation of slip during flow of a polymer melt past a solid surface depends on several parameters, such as film thickness, the strength of polymer–solid interactions compared to the cohesive energy of the polymer, and the roughness of the surface. Understanding the role of these molecular aspects for slip is crucial in microfluidics, friction-tuning, polymer extrusion, and nanocomposites applications. The present article investigates the effect of surface nanopatterning on slip, via Couette-flow simulations of long chain polyethylene melts past nanopatterned gold surfaces. Slip is quantified in terms of the true and effective slip velocity, and the slip length. When polymer chains are adsorbed to surfaces with periodic features (e.g., crystal planes), they develop preferential ordering in a way that enables them to minimize their free energy. The orientation of a chain is affected by that of its neighbors; thus, when several chains come together, they are prone to form regions with crystal-like orientation. We show that, in some cases, the introduction of nanopatterns on the surface can perturb and induce reorganization of these regions, and in turn affect slip. The nanopatterns are realized as periodic defect stripes of variable width, depth, areal density, and orientation angle. In situations in which the width of the defects becomes comparable to the diameter of individual chain backbones, slip is minimized (stick conditions). Cutting the nanopatterns in low symmetry directions can affect the quality of their edges and lead to enhanced friction. To characterize these edges we have devised a scheme for the quantification of the mean square roughness and mean position of the surface, which is general and applicable in 2 and 3 dimensions for any kind of material, either crystalline of amorphous. Applying the patterns on the opposing solid surfaces in a symmetric or antisymmetric manner has a profound effect on flow. We show that the application of nanopatterns in symmetric configurations generates zero net flow and induces additional shear along directions normal to the direction of the flow. The application of symmetry-breaking configurations can guide flow toward preferential directions, a result with possible applications in microfluidic devices.

聚合物熔体流经固体表面时的滑移（slip）现象，其表现受多项参数影响，包括膜厚、相较于聚合物内聚能（cohesive energy）的聚合物-固体相互作用强度，以及固体表面粗糙度。明晰上述分子层面因素在滑移过程中的作用，对微流控、摩擦调控、聚合物挤出及纳米复合材料等应用领域至关重要。
本文通过对长链聚乙烯熔体流经纳米图案化金表面的库埃特流动（Couette-flow）模拟，研究表面纳米图案化（nanopatterning）对滑移行为的影响，并通过真实滑移速度、有效滑移速度以及滑移长度（slip length）三个指标对滑移行为进行量化表征。
当聚合物链吸附于具有周期性结构（如晶面（crystal planes））的表面时，链会形成择优取向，以此实现自身自由能（free energy）的最小化。单条聚合物链的取向会受相邻链取向的影响；因此，当多条链聚集时，易形成具有类晶取向的区域。
研究表明，在部分场景中，表面引入纳米图案可扰动并诱导上述类晶取向区域发生重组，进而对滑移行为产生影响。本研究中的纳米图案为周期性缺陷条纹，其宽度、深度、面密度以及取向角均可调节。当缺陷宽度与单条聚合物链主链直径相当之时，滑移现象将被显著抑制，即达到黏附条件（stick conditions）。沿低对称方向制备纳米图案，会劣化图案边缘的质量，进而导致摩擦加剧。
为表征此类边缘特征，我们开发了一套用于量化表面均方粗糙度（mean square roughness）与平均位置的方法，该方法具有普适性，可适用于二维与三维空间下任意晶态或非晶态材料的表面分析。在相对的两个固体表面分别施加对称或反对称排布的纳米图案，会对流体流动产生显著影响。
研究发现，采用对称排布的纳米图案时，流体净流量为零，且会在垂直于流动方向的方向上诱导产生额外剪切力。而采用打破对称的排布方式，则可引导流体沿择优方向流动，该结果有望应用于微流控器件开发。