Frequency-dependent Ultrasonic Stimulation of PNIPAM Microgels in Water

TL;DR

This study demonstrates that high-frequency ultrasonic waves can induce the collapse and agglomeration of PNIPAM microgels in water by breaking hydrogen bonds, with effects depending on frequency, concentration, and duration.

Contribution

It introduces a novel ultrasonic stimulation method to control PNIPAM microgel behavior in water, highlighting frequency-dependent effects on hydrogen bond disruption.

Findings

Higher ultrasonic frequency accelerates microgel collapse.

Solution concentration influences the rate of turbidity increase.

Ultrasound energy absorption correlates with stimulation speed.

Abstract

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.