| Forces acting between surfaces define, in part, the behavior and stability of colloidal systems. A better understanding of those forces has traditionally been key in the development of subjects such as the behavior of colloidal suspensions, adhesion, foams, lubrication, and wetting. Recently, the advent of microfluidic devices and microelectromechanical systems (MEMS) has put new emphasis on surface forces. In particular, the ability to tune interfacial properties such as wetting and adhesion is of great importance in those devices. The technological need for improved understanding of external control of surface forces served as incentive to develop the surface forces apparatus (SFA) to allow external control of the surface potential. We developed the electrochemical surface forces apparatus (ESFA) by replacing the typical mica surface with a specifically designed gold electrode. The gold electrode was connected to a potentiostat in a three electrode arrangement allowing a variety of electroanalytical measurements to be performed inside the SFA.; Using the ESFA, potential dependent double layer forces were measured between the gold electrode and the mica surface in KClO4 solutions. By changing the applied potential on the gold surface, a large range of surface potentials was covered. Notably, the double layer forces saturated at the most negative potentials. Long-range electrostatic double layer forces were well predicted by standard double layer theory, but strong deviations were observed at short-range.; Attractive forces between gold and mica were characterized by measuring pull-off forces. These forces were strongly dependent on applied potentials. A model based on electrocapillary thermodynamics was developed to predict the potential dependence of adhesion. Application of this model displayed evidence of charging at the gold-mica interface.; The effect of potential dependent adsorption on double layer forces and adhesion was studied in pyridine solutions. The potential of zero force was shown to be a governing parameter. Pyridine trapped at the interface was found to be a likely factor in the electrocapillary behavior observed. Finally, it was shown that double layer forces can be used to actively make or break contact between two interacting surfaces. The development of the ESFA has opened the door to studying a wide range of new experimental systems. |
