Imbibition is defined as the displacement of one fluid by another immiscible fluid. This process is controlled and affected by a variety of factors. In spontaneous imbibition of wetting liquids into porous media (also called wicking), the capillary pressure, created as a result of interplay of the liquid and solid surface energies, is responsible for the spontaneous suction of the liquids. The capillary number (Ca) and the mobility ratio (M) have the greatest importance. It is also defined as the phenomenon by which the living or dead plant cell absorbs water by surface attraction. A classification of imbibition processes was given by Payatakes and Dias:
In a two-phase flow in porous media, imbibition is one of the two types of displacement, the other one being drainage. 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 and should not be confused.
- Spontaneous imbibition
- Constant influx
- Quasi-static imbibition
- Dynamic invasion with constant flow rate of the displacing fluid
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, 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.