  The drying dynamics in three dimensional porous media are studied with
confocal microscopy. We observe abrupt air invasions in size from single
particle to hundreds of particles. We show that these result from the strong
flow from menisci in large pores to menisci in small pores during drying. This
flow causes air invasions to start in large menisci and subsequently spread
throughout the entire system. We measure the size and structure of the air
invasions and show that they are in accord with invasion percolation. By
varying the particle size and contact angle we unambiguously demonstrate that
capillary pressure dominates the drying process.
