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

Abstract

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 °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.

Bright Circularly Polarized Mechanoluminescence from 0D Hybrid Manganese Halides

“…[ 9a ] In addition, thermally populated nonradiative recombinations are inhibited during ML emission, as the crystal fracture interrupts the process of energy accumulation. [ 16 ] Thus, the ML emission peaks of R ‐1 show negligible shifts (<2 nm) with increasing temperature (Figure 3c), indicating the absence of significant thermally populated vibrational states during ML emission. [ 17 ] As known, thermally populated vibrational states usually lead to nonradiative recombinations in the emission process.…”

Ultrasound‐Induced Adsorption of Acousto‐Responsive Microgels at Water–Oil Interface

“…The second effect arises from the volume phase transition of microgels due to the ultrasound (the collapsed microgels in bulk and at the interface). In previous studies, [ 18,19 ] we demonstrated that the absorbed energy from ultrasound by the liquid is able to break hydrogen bonds between linear PNIPAM and surrounding water molecules. In the current study, it appears that the PNIPAM microgels are responsive to ultrasound as well.…”

“…[ 15 , 16 , 17 ] We recently showed that ultrasound emerges as novel and interesting stimulus for the volume phase transition (VPT) of PNIPAM. [ 18 , 19 ] PNIPAM microgels are known for their temperature‐responsive behavior in water, with a volume phase transition temperature (VPTT) of approximately 32 °C, [ 1 ] the same as lower critical solution temperature (LCST) of linear PNIPAM at low solution concentrations. [ 20 ] PNIPAM has been studied intensively in bulk and at the interface for more than three decades now.…”

“…In particular, being able to induce the VPT of microgles by ultrasound at the interface may lead to novel applications in various fields, e.g., pharmacology, food industry or interfacial catalysis. [ 19 , 32 ] By shrinking microgels after adsorption, embedded payload material can be released at the interface, allowing for controlled and targeted delivery or catalysts. It has to be emphasized that here ultrasound is not destructive since it has much higher frequencies (MHz range) and lower power amplitudes (W range) compared to typical ultrasound baths that operate in kHz range with power amplitude in the range of hundreds of Watts.…”

Impact of Ultrasound on the Motion of Compact Particles and Acousto-responsive Microgels

“…One of the external elds that have drawn attention in recent decades is US which is employed to manipulate the physical or chemical properties of particles and molecules with applications in drug delivery [14], catalysis [15] and materials synthesis [16]. Recently, we showed that high-frequency US, in its non-destructive condition (low amplitude), can be used as a stimulus to induce a phase transition in solutions of linear poly(N-Isopropylacrylamide)(PNIPAM) [17,18]. The dehydration of PNIPAM which usually triggered by increasing the temperature above the lower critical solution temperature (LCST), is then induced by US.…”