Frequency-Dependent Ultrasonic Stimulation of Poly(N-isopropylacrylamide) Microgels in Water

As a novel stimulus, we use high-frequency ultrasonic waves to provide the required energy for breaking hydrogen bonds between Poly(N-isopropylacrylamide) (PNIPAM) and water molecules while the solution temperature is maintained below the volume phase transition temperature (VPTT = 32 degrees C). Ultrasonic waves propagate through the solution and their energy will be absorbed due to the liquid viscosity. The absorbed energy partially leads to the generation of a streaming flow and the rest will be spent to break the hydrogen bonds. Therefore, the microgels collapse and become insoluble in water and agglomerate, resulting in solution turbidity. We use turbidity to quantify the ultrasound energy absorption and show that the acousto-response of PNIPAM microgels is a temporal phenomenon that depends on the duration of the actuation. Increasing the solution concentration leads to a faster turbidity evolution. Furthermore, an increase in ultrasound frequency leads to an increase in the breakage of more hydrogen bonds within a certain time and thus faster turbidity evolution. This is due to the increase in ultrasound energy absorption by liquids at higher frequencies.

Automated summary

This study investigates the use of high-frequency ultrasound to stimulate PNIPAM microgels and break the hydrogen bonds between water molecules. The absorption of ultrasound energy leads to the collapse of microgels and an increase in solution turbidity. The increase in solution concentration and ultrasound frequency accelerates the evolution of turbidity.