Rudge Raisa E D, van de Sande Jesse P M, Dijksman Joshua A, Scholten Elke
Physics and Physical Chemistry of Foods, Wageningen University, The Netherlands.
Physical Chemistry and Soft Matter, Wageningen University, The Netherlands.
Soft Matter. 2020 Apr 15;16(15):3821-3831. doi: 10.1039/d0sm00080a.
Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials.
滚动球轴承广为人知，并用于减少两个表面之间的摩擦。最近，水凝胶-水凝胶摩擦副也展现出良好但相当复杂的润滑特性。在此，我们用水凝胶作为球轴承来阐明软球形颗粒具有与速率相关的非平凡润滑行为。与牛顿润滑或干固体摩擦不同，悬浮液中的水凝胶颗粒会根据滑动速度经历四种摩擦状态。我们将不同状态与颗粒在不同间隙尺寸下的变形联系起来，这会改变滑动表面之间的有效接触面积。通过系统地改变颗粒特性和滑动表面的性质，我们确定了每种不同润滑状态随速度变化的潜在机制：（I）由于颗粒扁平化和相互作用物体之间的直接接触导致相对较高的摩擦；（II）由于滚动颗粒的存在导致摩擦减小；（III）在受限空间中大量颗粒流入导致颗粒压缩；（IV）形成厚润滑层。使用这些含有软的、可变形颗粒的悬浮液作为球轴承系统，我们为软材料摩擦提供了新的见解，这些见解可应用于乳液、粉末、糊剂或其他颗粒材料。
