CAVITATION DURING DRYING OF A GEL

The capillary tension developed during drying of a gel is so large that cavitation (homogeneous nucleation of bubbles of vapor) could occur. It is shown that homogeneous nucleation is possible only if a certain characteristic pore size (the breakthrough radius) is small enough to generate capillary tension exceeding a critical value predicted by classical nucleation theory. Since the criterion for bubble growth is the same as for advance of menisci into the drying surface, bubbles can reach macroscopic dimensions only after the critical point of drying (CPD). Heterogeneous nucleation can occur at any time, but growth of the bubbles is still possible only after the CPD is reached. Nucleation is facilitated when: (a) the pore entry radius is much smaller than the pore interior; (b) there is a strong gradient in composition, so that the liquid/vapor surface tension is highest near the drying surface; or (c) there is a spatial gradient in pore size, with the smallest pores near the drying surface. Nitrogen desorption measurements can be used to determine the highest tension reached during drying of a xerogel; in this way it is shown that capillary tension can approach the values necessary to produce homogeneous nucleation.