ABSTRACT
When the applied pressure is reduced, as in the latter case, the meniscus withdraws from the cavity. The relevant contact angle is now θr, the receding angle, as shown in Fig. 18b.
The concept of the threshold size is also applicable here. The pressure inside and outside the bubble are related through the Laplace equation:
In the case where the contact angle is exactly θa when the external pressure P is imposed, the cavity is a threshold cavity, and
The situation is shown in Fig. 19. The opening of the conical pit has a radius r0. The conditions (contact angle, radius of contact) are such that the cavity is “subthreshold,” (A), “threshold,” (B) or “superthreshold.”
