Induction of foaming in vacuum drying by needle stimulation and the impact of solution viscosity, vapor pressure, and the type of solute and solvent

Reliable induction of foaming is indispensable for vacuum foam drying. We previously showed that, adequate partial drying followed by puncturing a solution with a needle (needle stimulation) resulted in the induction of the foaming, even under insufficient vacuum where foaming is minimal. Herein, characteristics of needle-stimulation-induced foaming was investigated, focusing on the impact of solution viscosity and vapor pressure. Different combinations of solutes (disaccharides, polyvinylpyrrolidones, polyacrylic acid, Eudragit) and solvents (methanol, ethanol, isopropanol, water) were used. Needle stimulation was conducted after different periods of initial vacuum drying, and the probability of foaming induction during the restart of vacuum drying was determined. Each series of solutes (polymers and disaccharides) respectively indicate solvent-independent correlations between foaming induction probability and solution viscosity. The existence of a lower limit for the solution vapor pressure required for foaming induction was also implied. A possible mechanism for needle-stimulation-induced foaming under reduced pressure is proposed.