Confining Liquids on Silicon Surfaces to Lubricate MEMS

Liquid lubrication may provide a solution to the problem of high friction and wear in micro-electromechanical systems. Although the effectiveness of this approach has been demonstrated in laboratory-based friction tests, practical constraints prevent it from being applied in commercial devices. The main problem is how to position the lubricant on a silicon surface in order to limit spreading and evaporation. This paper describes two techniques to address this issue. First, low concentrations of additives are used to promote autophobic behaviour. Tests’ results show that certain concentrations of both multiply alkylated cyclopentane and amine additives are effective in halting the spread of hexadecane on silicon, and, in the latter case, cause the hexadecane drop to subsequently retract. The second approach involves applying a micro-contact printing technique previously used on gold surfaces. Here, silicon surfaces are coated with octadecyltrichlorosilane mono-layers that are then selectively removed, using oxygen plasma, to leave regions of contrasting surface energy. Results from spin tests show that surfaces treated in this way can anchor 1 ll drops of hexadecane and water when forces of up to 22 and 230 lN, respectively, are applied.