gel
A gel is the clear, wobbly, almost-solid medicine that holds its shape in the tube but spreads thin and smooth on the skin — think of the texture of hair gel or a fruit jelly. It is mostly liquid, yet it does not flow, which seems contradictory until you see what is happening at the microscopic level.
A gel is a semisolid system in which a large amount of liquid is trapped inside a continuous three-dimensional network of a gelling polymer or other colloidal particles. The network gives elasticity and structure, holding the liquid the way a sponge holds water, so the whole mass behaves as a soft solid even though it is mostly solvent. Common gelling agents include carbomers, cellulose derivatives and natural gums.
Most pharmaceutical gels are aqueous (hydrogels), so they feel cool, non-greasy and wash off easily, and they leave a transparent film on drying — well suited to hairy or oily areas where ointments are unwelcome. The water phase means a preservative is needed, and a hydrogel can deliver only water-compatible drugs; the gel can also dry out or lose structure if the polymer network is disrupted by pH or electrolytes.
Diclofenac gel rubbed over an arthritic knee delivers an anti-inflammatory drug locally through the skin, the cool, non-greasy carbomer hydrogel being more acceptable than a greasy ointment.
A polymer network turns a mostly-liquid system into a spreadable soft solid.
A gel is mostly liquid yet behaves like a soft solid because of its internal polymer network; many gels are also thixotropic — they thin and flow when squeezed or rubbed, then re-set when left alone.