Imbibition is a special type of diffusion when water is absorbed by solids-colloids-causing them to enormously increase in volume. The classical examples of imbibition are absorption of water by seeds and dry wood. The pressure that is produced by swelling of wood had been used by the prehistoric man to split rocks and boulders. If it were not for the pressure due to imbibition, seedlings would not have been able to emerge out of soil into the open; they probably would not have been able to establish.
Imbibition is also diffusion since water movement is along a concentration gradient; the seeds and other such materials have almost no water hence they absorb water easily. Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition. In addition, for any substance to imbibe any liquid, affinity between the adsorbant and the liquid is also a pre-requesite.
Imbibition occurs when a wetting fluid displaces a non-wetting fluid, contrary to drainage where a non-wetting phase displaces the wetting fluid. The two processes are governed by different mechanisms.
One example of imbibition that is found in nature is the absorption of water by hydrophilic colloids. Matrix potential contributes significantly to water in such substances. Examples of plant material which exhibit imbibition are dry seeds before germination. Imbibition can also entrain the genetic clock that controls circadian rhythms in Arabidopsis thaliana and (probably) other plants. Another example is that of imbibition in the Amott test.
Different types of organic substances have different imbibing capacities. Proteins have a very high imbibing capacity then starch less and cellulose least. That is why proteinaceous pea seeds swell more on imbibition than starchy wheat seeds.
Imbibition of water increases the volume of the imbibant, which results in imbibitional pressure. This pressure can be of tremendous magnitude. This fact can be demonstrated by the splitting of rocks by inserting dry wooden stalks in the crevices of the rocks and soaking them in water, a technique used by early Egyptians to cleave stone blocks.
Skin grafts (split thickness and full thickness) receive oxygenation and nutrition via imbibition, maintaining cellular viability until the processes of inosculation and revascularisation have re-established a new blood supply within these tissues.