Influence of cellulose fibers and fibrils on nanoscale friction in kraft paper

Friction affects the use of paper in many ways, from the manipulation and slippage control of paper sheets in large industrial machines to rolling and inking transfer in ballpoint pens. Paper is mainly constituted of cellulosic structures, whose contribution to friction is still obscure. In this work, the contribution of cellulose fibers, macrofibrils, and microfibrils to the forces exerted on a nanoscale tip as it moves across the paper surface is addressed. The surface topography of monolucid kraft paper was measured by atomic- and friction-force microscopies. Cellulose fibers and entangled macro- and microfibrils were observed. Their aspect ratios were measured, and their contribution to friction with a nanoscale silicon tip was determined. The forces opposing the motion of the tip were attributed to tripping and intermolecular friction. The former was measured mainly at extended contacts at the lateral surface of cellulose fibers, while the latter was mainly observed at point contacts on top of macro- and microfibrils.