Institute of Neuroscience (IoNS), Université catholique de Louvain, Brussels, Belgium.
J R Soc Interface. 2011 Nov 7;8(64):1574-83. doi: 10.1098/rsif.2011.0086. Epub 2011 Apr 13.
The dynamics of fingertip contact manifest themselves in the complex skin movements observed during the transition from a stuck state to a fully developed slip. While investigating this transition, we found that it depended on skin hydration. To quantify this dependency, we asked subjects to slide their index fingertip on a glass surface while keeping the normal component of the interaction force constant with the help of visual feedback. Skin deformation inside the contact region was imaged with an optical apparatus that allowed us to quantify the relative sizes of the slipping and sticking regions. The ratio of the stuck skin area to the total contact area decreased linearly from 1 to 0 when the tangential force component increased from 0 to a maximum. The slope of this relationship was inversely correlated to the normal force component. The skin hydration level dramatically affected the dynamics of the contact encapsulated in the course of evolution from sticking to slipping. The specific effect was to reduce the tendency of a contact to slip, regardless of the variations of the coefficient of friction. Since grips were more unstable under dry skin conditions, our results suggest that the nervous system responds to dry skin by exaggerated grip forces that cannot be simply explained by a change in the coefficient of friction.
指尖接触的动力学表现为从粘滞状态向完全滑动状态转变过程中观察到的复杂皮肤运动。在研究这种转变时，我们发现它取决于皮肤的水合作用。为了量化这种依赖性，我们要求被试者在玻璃表面上滑动食指，同时借助视觉反馈保持相互作用力的法向分量恒定。用光学仪器对接触区域内的皮肤变形进行成像，使我们能够定量确定滑动和粘滞区域的相对大小。当切向力分量从 0 增加到最大值时，粘滞皮肤区域与总接触区域的比值从 1 线性减小到 0。这种关系的斜率与法向力分量成反比。皮肤水合作用水平显著影响从粘滞到滑动接触的动力学过程。具体影响是降低接触滑动的趋势，而与摩擦系数的变化无关。由于在干燥皮肤条件下抓握更不稳定，因此我们的结果表明，神经系统通过过度的抓握力来应对干燥的皮肤，而这种力不能简单地用摩擦系数的变化来解释。
