Mechanism for helical gel formation from evaporation of colloidal solutions

We use MRI imaging to decipher the physical mechanism behind helical gel formation when a colloidal solution is evaporated from a small vertical or inclined capillary. A gel column, surrounded by the solvent, is observed to appear in the middle of a capillary. For nearly vertical capillaries, the denser gel column buckles under gravity to form a loose spiral. Further heating leads to the formation of a helical vapor pocket, surrounded by asymmetric liquid menisci. As the heating continues, this vapor pocket propagates downward and traces the buckled column. If gravity buckling occurs and if the maximum thickness of the annular solvent film is less than the solvent capillary length, significant nonuniform vapor pressure builds up within the vapor bubble because the vapor's escape is obstructed and because the evaporation is nonuniform. This upward air pressure spiral is amplified by asymmetric menisci of the vapor pocket to produce a high liquid pressure gradient along the liquid spiral next to the helical vapor pocket. Both pressures are inversely proportional to the internal capillary diameter d, and together, they twist the buckled column to a much higher pitch. The balance of this force to the elastic force of the buckled column, which opposes coiling, leads to a minimum distance between pitches L that scales as d. When all the fluid outside the column has evaporated, the slow vapor release by the drying gel cannot provide sufficient pressure for coiling. Hence, the "compressed spring" starts to rewind and lengthen. The dominant force balance with the opposing dry friction force leads to a lower final pitch with an L similar to d(2) scaling. Both these scalings are consistent with our experimental data.