Figure 10. Part of the radiation force on the bilayer is due to a direct interaction with ultrasound.

A. Mean (±SE) capacitive current response amplitude as a function of the thickness of the solution layer between the bilayer and the solution/air interface, normalized to the response amplitude at the reference thickness (1.7 mm, corresponding to the standard solution volume of 300 µL in the upper compartment of the chamber) and to the bilayer capacitance (see Materials and Methods, “Data Analysis” section), for ultrasound at 1 MHz and 610 mW/cm² (n = 2–8). The solid blue line is an exponential fit with amplitude of 2.7±0.2, exponential decay constant of 0.84±0.14, and baseline value of 0.38±0.08 (parameters ± SD). B. As in A, but for focused ultrasound at 43 MHz and 13 W/cm² (intensity at focal spot, located on the bilayer) (n = 3–8). The solid blue line is an exponential fit with amplitude of 17±14, exponential decay constant of 2.3±0.8, and baseline value of 0.53±0.10 (parameters ± SD). C. Currents in response to ultrasound at 43 MHz and 13 W/cm², under voltage-clamp at −100 mV, with an ∼8 mm thick solution layer between the bilayer and the solution air interface (blue current trace), and in response to the same ultrasound stimulus with the reflective solution/air interface replaced by an acoustic absorber (red current trace). The red current trace is the average of 10 ultrasound applications, and the blue current trace is the average of the currents before putting the acoustic absorber in place and after removing it, each the average of 10 ultrasound applications. The capacitance of the bilayer was 180 pF. D. Mean (±SE) capacitive current response frequency (red circles) and damping constant (blue circles) as a function of the thickness of the solution layer between the bilayer and the solution/air interface, for ultrasound at 1 MHz and 610 mW/cm² or at 43 MHz and 13 W/cm² (combined data sets from A and B, n = 9–15). The solid lines are linear fits with slope and intercept of −14±2 Hz/mm and 480±9 Hz (frequency), and 2.7±4.3 s⁻¹/mm and 440±20 s⁻¹ (damping constant) (parameters ± SD).