Many bio‐hydrogels are prepared thanks to the addition of divalent ions. These gels usually exhibit partial ionic selectivity, a feature is leveraged to instigate an osmotic flow during the gelation process across a range of bio‐hydrogels. A simple experimental setup consisting a glass capillary filled by the reactant solution is taken as an advantage, brought into contact with a reservoir of calcium chloride. Direct observation allows to characterize the gelation kinetics, the permeability and the selectivity of several gels, including some made of protein aggregates, pectin, and alginate. It is shown that these intrinsic properties, coupled with appropriate gelation kinetics, intricately govern the osmotic flow induced by the chemical potential difference of the calcium chloride ions imposed during gel formation, consistent with the Kedem–Kashalsky equation. The forming gel acts as a semi‐permeable membrane for calcium chloride ions. The consequences of this osmotic flow are of potentially great interest since it triggers an increase of the gel concentration close to its boundary, opening the road to their spontaneous structuration. The formation of this dense shell is well accounted for the combination of a mass balance and a kinetic model.