Measuring the effects of a pulsed excitation on the buildup of acoustic streaming and the acoustic radiation force utilizing an optical tweezer

Pulsed excitations of piezoelectric transducers affect during the buildup the force contributions from acoustic streaming (AS) and the acoustic radiation force (ARF) to the total force in a standing pressure wave differently. We find with an optical tweezer as measuring instrument that during the first 120 000 excitation periods and across different pulsing frequencies, the AS-induced displacement is on average less than 20% of its nonpulsed value for a duty cycle of 50%, whereas the ARF-induced displacement is around 50%. These findings show that a pulsed excitation can be a tool for reducing AS compared to the ARF.

Figure 1

Schematic of device and buildup measurement setup.

Figure 2

Photograph of optical trapping setup with highlighted important equipment.

Figure 3

Photograph of acoustofluidic device with highlighted parts.

Figure 4

Schematic of optical trapping setup, laser settings ($P$: laser power), and optical shutter settings ($T$: transmittance for ${\lambda }_{\mathrm{L}}=785\mathrm{nm}$) before and after the buildup measurement (BM; top) and during the BM (bottom). During the BM the particle is free-floating, the laser power is reduced as low as possible, the shutter is opened to allow optical position detection on the quadrant photo detectors (QPDs), and the ultrasound is switched on; hence an acoustic force $F_{\mathrm{ac}}$ acts on the particle. Before and after the BM all states are switched to their respective opposite.

Figure 5

Process diagram for one measurement cycle. The gray rectangles represent the processes during the buildup measurement (BM). During this time the particle is free-floating (orange area). All other process steps are before or after a BM. The time stamps in the hatched rounded rectangles represent the times when the process attached to them is executed. One measurement cycle takes about 2 s to be executed. The BM itself is less than $100\mu \mathrm{s}$.

Figure 6

Schematic of pulsed acoustic excitation for three different duty percentages (50%, 70%, and 100%) over pulsing cycles $t{f}_{\mathrm{p}}$ with the relation $f_{\mathrm{ex}}=k{f}_{\mathrm{p}}$ (here $k=10$).

Figure 7

Evolution of averaged voltage differences $\mathrm{\Delta }{V}_y$ (left column; ARF associated) and $\mathrm{\Delta }{V}_z$ (right column; AS associated) for three different pulsing frequencies $f_{\mathrm{p}}$ and different duty percentages at $\mathrm{\Delta }y=-0.4\mathrm{mm}$ over excitation periods ${}^t{/}_{t_0}$ with $t_0={}^1{/}_{f_{\mathrm{ex}}}$. For the first row $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{1000}$, for the second $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{5000}$, and for the last row $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{10000}$. The data per plot are normalized to the maximal amplitude of the data series with a duty percentage of 100%. The gray-shaded area of each plot marks the time when the US is off. Additionally, the vertical solid lines in the bottom left plot mark the beginning of a new pulsing cycle.

Figure 8

Evolution of averaged voltage differences $\mathrm{\Delta }{V}_y$ (left column; ARF associated) and $\mathrm{\Delta }{V}_z$ (right column; AS associated) for three different pulsing frequencies $f_{\mathrm{p}}$ and different duty percentages at $\mathrm{\Delta }y=-0.5\mathrm{mm}$ over excitation periods ${}^t{/}_{t_0}$ with $t_0={}^1{/}_{f_{\mathrm{ex}}}$. For the first row $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{1000}$, for the second $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{5000}$, and for the last row $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{10000}$. The data per plot are normalized to the maximal amplitude of the data series with a duty percentage of 100%. The gray-shaded area of each plot marks the time when the US is off. Additionally, the vertical solid lines in the bottom left plot mark the beginning of a new pulsing cycle.

Figure 9

Evolution of averaged voltage differences $\mathrm{\Delta }{V}_y$ (left column; ARF associated) and $\mathrm{\Delta }{V}_z$ (right column; AS associated) for three different pulsing frequencies $f_{\mathrm{p}}$ and different duty percentages at $\mathrm{\Delta }y=-0.6\mathrm{mm}$ over excitation periods ${}^t{/}_{t_0}$ with $t_0={}^1{/}_{f_{\mathrm{ex}}}$. For the first row $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{1000}$, for the second $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{5000}$, and for the last row $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_{10000}$. The data per plot are normalized to the maximal amplitude of the data series with a duty percentage of 100%. The gray-shaded area of each plot marks the time when the US is off. Additionally, the vertical solid lines in the bottom left plot mark the beginning of a new pulsing cycle.

Figure 10

Ambient temperature $T$ at device for $k=1000$ BM series over time. The experiment of 80% occurred between 14:00 and 15:00 (gray-shaded area).

Figure 11

Final normalized position at end of the buildup measurement of $\mathrm{\Delta }{V}_y$ (left column; ARF associated) and $\mathrm{\Delta }{V}_z$ (right column; AS associated) over duty percentage for the three pulsing frequencies $f_{\mathrm{p}}={}^{f_{\mathrm{ex}}}{/}_k$ and averaged over the three locations $\mathrm{\Delta }{y}_i=\{-0.06\mathrm{mm},-0.05\mathrm{mm},-0.04\mathrm{mm}\}$. The outlier for $k=1000$ and 80% duty width is attributed to an unknown external disturbance within the OT laboratory.