Individual molecules move around in three-dimensional space in a process known as diffusion. Their speed depends on many factors, such as the viscosity and temperature of the medium through which they move — for example, whether through the air or a coffee slurry. But in spite of the seemingly random movement of each individual particle, there is an overall tendency for the molecules in a mix to migrate from areas of higher concentration towards areas of lower concentration.
Because coffee grinds include so much soluble material, a coffee slurry always contains an uneven distribution of molecules. Water adjacent to a coffee grind is likely to contain a higher concentration of flavour molecules than does water at a farther distance from the coffee grind. The unevenness of concentration in a slurry creates the chemical potential for diffusion to occur. When a region of high concentration borders a more dilute region, the term ‘concentration gradient’ describes the difference between them. ‘Diffusion’ describes the movement of molecules from an area of higher concentration towards one of lower concentration. The process of diffusion doesn’t require any additional energy; all it requires is a concentration gradient.