Measurement of electrophoretic forces and the kinetics thereof with an AFM cantilever

AFM cantilevers were used to probe electrophoretic forces while switching an external electric field. As expected, the forces were in the range of a few 10´s of piconewtons. They could be easily resolved with the optical lever technique, provided that data were accumulated and averaged for a few minutes. The time resolution of the experiment was 200 microseconds. The kinetics were rather complicated. There was a fast movement of the negatively charged cantilever towards the positively charged electrode, which in most cases was not kinetically resolved and in other cases lasted a few milliseconds. This first step was followed by a reverse motion, which in most cases could be interpreted as a relaxation. The time scale of this motion roughly matched the time scale, over which the electric current through the electrodes decayed. The electric current was mostly due to double layer recharging, possibly accompanied by some charge transfer at the electrodes. The current caused a decay of the electric field at the cantilever, hence the decay of the cantilever deflection. Interestingly, the second stage occasionally drove the cantilever deflection to the opposite side. The second process in these cases overcompensated the first process, which is in conflict with the notion of this second process being entirely caused by a relaxation. The cantilever never came to rest over a few milliseconds. The origin of the slow movement in the final stage of the modulation cycle may be rooted in convection.