  As a novel stimulus, we used 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 maintains below the
volume phase transition temperature (VPTT=$$32^\circ C$$). 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 the water and
agglomerate, resulting in turbidity. We used turbidity to quantify the
ultrasound energy absorption and showed 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 hydrogen
bond breakage and turbidity evolution. Furthermore, the frequency of imposed
waves is important and affects the stimulation kinetics of PNIPAM microgels.
Increasing the frequency of actuation increases the speed of hydrogen bond
breakage and thus turbidity evolution. This is due to the increase in
ultrasound energy absorption by liquids at higher frequencies.
