Intrusion pressure to initiate flow through pores between spheres

In this work, clusters of three or four spheres were used to examine intrusion pressure. Polytetrafluoroethylene (PTFE) or polyamide 66 (PA66) spheres were arranged horizontally to create a single pore. Liquid drops of water or ethylene glycol were gently introduced from above. If the spheres were too large, drops flowed through as soon as they were deposited. If the spheres were too small, liquid was suspended in the neck of the pore and could not pass through; drops became unstable and fell to one side. Alternatively, if spheres of a certain size were chosen, then capillary forces initially prevented drops of lesser stature from breaking though. However, as these drops grew taller, they eventually reached a height where the gravitational force exceeded the capillary force and the liquid flowed through the pore. A simple model for intrusion pressure was derived. Estimates from the model agreed well with experimentally measured values.