Visualization of Acoustic Energy Absorption in Confined Aqueous Solutions by PNIPAM Microgels: Effects of Bulk Viscosity

Abstract

Ultrasound propagation in liquids is highly influenced by its attenuation due to viscous damping. The dissipated energy will be partially absorbed by the liquid due to its dynamic viscosity as well as its bulk viscosity. The former results in the generation of a flow that is called acoustic streaming, and the latter is associated with the vibrational and rotational relaxation of liquid molecules. Measuring the ultrasonic wave attenuation due to the bulk viscosity is presented as a novel method in this article. Poly(Nisopropylacrylamide) (PNIPAM) microgels, which are soluble in several solvents such as water, were used as acousto-responsive markers in water, which upon absorption of ultrasonic energy undergo a volume phase transition due to the breakage of their hydrogen bonds. Thus, they become insoluble in water, and due to shrinking, their optical density increases. As a result, their agglomeration can be seen as a turbid medium. We managed to visualize the ultrasonic energy absorption due to the bulk viscosity using the turbidity since the excess acoustic energy on top of the absorbed energy for the translational motion of liquid is spent to break the hydrogen bonds between PNIPAM and water. In addition, to quantify the turbidity phenomenon, the total energy required for breaking hydrogen bonds in the solution is calculated, and its evolution, according to the input power intensity, is quantified by image processing. The effect of viscosity by changing the microgel concentration was investigated, and it is shown that an increasing microgel concentration increases the acoustic energy absorption rate much greater than its dynamic viscosity. Therefore, the bulk viscosity, as the responsible parameter for this increase, is measured directly from the energy of broken hydrogen bonds. The results show that at low solution concentration (0.2 wt %) the bulk viscosity is in the same order of magnitude as its dynamic viscosity. Increasing the concentrations to 1 and 5 wt % increases the bulk viscosity and consequently the structural relaxation time by 1 and 2 orders of magnitude, respectively.

Synthetic Thermo-Responsive Terpolymers as Tunable Scaffolds for Cell Culture Applications

“…This is due to intermolecular aggregation caused by the LCST effect, which depends on the polymer concentration. Beyond LCST, there is a higher phase separation that is unstable as temperature increases [ 80 ].…”

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

“…The kinetics of the energy absorption has been quantified similarly to our previous work [ 15 ] using image processing and the absorbed energy over time for different solution concentrations (i.e., 0.2, 1, and 5 wt%) is shown in Figure 2 . Increasing the solution concentration increases the ultrasound energy absorption and consequently increases the speed of turbidity evolution.…”

“…Increasing the solution concentration increases the ultrasound energy absorption and consequently increases the speed of turbidity evolution. In our previous work [ 15 ], it is shown that increasing the solution concentration leads to a greater increase in “bulk viscosity” that reflects the vibrational and rotational degrees of molecular freedom in comparison with the increase in “shear viscosity” that corresponds to their translational movement. Therefore, in denser solutions, the PNIPAM chains are slower in the structural reorganization of their molecules upon ultrasonic actuation and thus the energy absorption is larger.…”

“…In all of these cases, one has to modify the PNIPAM microgel by copolymerization so it becomes responsive to another stimulus. Recently, we have shown that PNIPAM microgels, without any additional modification, are acousto-responsive [ 15 ]. Hydrogen bonds between PNIPAM and water molecules will be broken due to the absorption of a certain amount of energy carried by ultrasound waves while the solution temperature is below VPTT.…”

“…As a result, the solution becomes turbid due to the agglomeration of the collapsed microgels. In our recent work [ 15 ], we showed that PNIPAM microgels are responsive to ultrasound energy, and by increasing the input power, turbidity evolution becomes faster. The question remains on the effects of input frequency on turbidity evolution (i.e., hydrogen bond breakage) of PNIPAM solutions.…”