Indexed on: 05 Dec '08Published on: 05 Dec '08Published in: Langmuir
The contact dynamics between a soft sphere and a rigid substrate on the micron scale was studied experimentally. The time evolution of the contact radius, contact angle, and the force acting on the sphere were measured simultaneously in the loading and the unloading cycle. There is little effect of repetition: the experimental results obtained in the second and third cycles agree completely with those of the first cycle. The contact angle changes dynamically in the loading and the unloading processes, and there are regions where the advancing angle and the receding angle remain constant. The experimental results were analyzed in terms of the extended Johnson-Kendall-Roberts theory, and it was found, to our surprise, that the theory works well: the theory predicts the force curve quite accurately if the apparent surface energy obtained from the contact radius is used. The apparent surface energy was experimentally obtained as a function of the contact line velocity, and it was found that (1) the curve agrees qualitatively with that predicted by Greenwood and Johnson, and (2) certain modification is needed when the velocity of the contact line changes the sign.