Expression: Expression or pressing is a process in which liquid contained in pores in a solid matrix or contiguous array of solid matrices or particles is expelled by compaction or squeezing. The pressed mixture is often called a “presscake”, or “cake” for short. Mechanically-, hydraulically- and pneumatically-driven moving surfaces provide compaction in most expression equipment. Incidental compaction of solid-fluid mixtures also causes expression. Fluid is pressed out of moist soils when added soil or structures are placed on top of them. If gas pressure acts on a soft cake whose pores are so fine that capillary “suction” pressure exceeds the applied pressure, the gas won't displace fluid directly: but the applied pressure will compact the cake and express fluid.