Electrostatic forces and micromanipulator design: on the importance of surface topography parameters

Micro manipulations of objects between 10 mum and 1 mm by contact are often disturbed by the surface forces appearing between the handled object and the gripper. They may overcome gravity and prevent the release of the object. Capillary, electrostatic and van der Waals forces are the main surface forces responsible for this adhesive phenomenon. Several factors may influence these forces such as the materials in contact, the fabrication process, the surface treatments or the surface contaminations. All of these contribute to shape the topography of the surface. In this paper is investigated in which way surface roughness could be used to overcome the problem of adhesive electrostatic forces. Simulations are performed for smooth surfaces and for rough surfaces in order to study the influence of surface topography. The simulation tool is validated using analytical models for the smooth case and shows a good correlation. Roughness is modeled using a fractal representation (Weierstrass-Mandelbrot function). The simulations using fractal representation show a significant influence of the surface topography at small separation distances and correlate experimental benchmarks from literature. Topography of the surface should not be neglected in micromanipulator design.