Temporal evolution of thermoacoustic streaming around a spatially confined temperature gradient

The ability to precisely control the localization of individual cells or other microscopic biological particles in aqueous suspension inside a microfluidic confinement is essential for the realization of future tools for single-cell studies in basic biology and medicine. We previously demonstrated that thermoacoustic streaming stemming from interactions of an acoustic field with a thermal gradient induced by focused light can produce fast, confined, and modular local flow fields. In this work, we study the buildup and decay of thermoacoustic streaming generated by a focused laser directed into an acoustic half-wavelength standing wave. For a pre-established sound field, we observe the timescale for buildup and decay of the thermoacoustic streaming upon activating the laser, and vice versa. For the system investigated, we find that while the buildup of thermoacoustic streaming takes only 2 ms for sound field-controlled thermoacoustic onset, laser-irradiation-controlled onset is ten times slower. This is explained by the relatively slow thermal diffusion in the system compared to the fast buildup of the acoustic resonance. Furthermore, we demonstrate how the combination of fast timing control by the sound field and the subsound-wavelength spatial control offered by the confined temperature gradient makes it possible to move a single particle a distance of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>100</a:mn> <a:mspace width="0.2em"/> <a:mtext fontfamily="times">μ</a:mtext> <a:mrow> <a:mi mathvariant="normal">m</a:mi> </a:mrow> </a:math> at <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mn>350</f:mn> <f:mspace width="0.2em"/> <f:mtext fontfamily="times">μ</f:mtext> <f:mrow> <f:mi mathvariant="normal">m</f:mi> </f:mrow> <f:mo>/</f:mo> <f:mrow> <f:mi mathvariant="normal">s</f:mi> </f:mrow> </f:math> while leaving the trajectory of another nearby particle, less than <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mn>25</l:mn> <l:mspace width="0.2em"/> <l:mtext fontfamily="times">μ</l:mtext> <l:mrow> <l:mi mathvariant="normal">m</l:mi> </l:mrow> </l:math> away, unaffected.