Study On Horizontal Irradiation Of Nanofluids By Pulsed Laser And Mechanism Of Regulating Bubble Motion

Microfluidic is an emerging technology in recent years.Due to its advantages such as low sample consumption,fast analysis speed,high degree of automation,miniaturization and integration,etc,microfluidic has been widely used in the fields of biology,detection,chemical engineering and control.As a simple,clean and effective tool,microbubble opens a new door to solve many difficulties in microfluidic technology.In the application of microfluidic technology based on microbubbles,it is very important to control the generation and motion of microbubbles.In this paper,the process of bubble motion controlled by pulsed laser is studied experimentally and theoretically.By using pulsed laser to irradiate the 100 ppm gold nanofluid horizontally,the indirect light pressure is coordinated with other forces to achieve the purpose of regulating bubble motion.Gold nanofluid was prepared by Frens sodium citrate reduction method.The gold nanofluid has high absorptivity to 527 nm laser.By adjusting the pulsed laser frequency f1 and the mono-pulse energy E1,there are four motion states of bubbles in the nanofluid.These four kinds of bubble motion states have remarkable characteristics respectively.In regime I,a large number of micro-nano bubbles are formed at the laser incidence and move quickly to the right.In regime ?,the trajectory of the bubble's centroid constitutes a complete triangle,and the motion process is controlled by the laser frequency.In regime III,the bubbles are suspended in the nanofluid stably and rotate rapidly in the counterclockwise direction.In regime IV,the bubble oscillates from side to side greatly driven by the laser and also rotates around its own center of mass.The bubble oscillation is not controlled by the laser frequency.Four flow patterns appear under different pulsed laser parameters.Regime ?(micro-nano bubble jet):the parameter ranges of laser frequency and single pulse energy are 50?2000 Hz and 0.05?4.00 mJ,respectively.Regime ?(the bubble triangular trajectory oscillates reciprocately under the control of pulsed laser frequency):the parameter ranges of laser frequency and single pulse energy are 50?500 Hz and 1.83?12.92 mJ,respectively.Regime ?(bubble rotation in a finite region):the parameter range of laser frequency and monopulse energy is 800?2000 Hz and 0.41?3.34 mJ,respectively.Regime ?(bubble low frequency horizontal oscillation):the the parameter range of laser frequency and monopulse energy is 100?2000 Hz and 2.09?12.40 mJ,respectively.The experimental conditions were drawn into a flow pattern zoning diagram,showing the characteristics of banded distribution.As the energy of the laser monopulse increases,the flow pattern of the bubbles changes from regime ? to regime ?(regime ?),and then from regime?(regime ?)to regime ?.In the middle band region,the flow pattern of bubble movement changes from flow pattern ? to flow pattern ? with the increase of laser pulse frequency.The coupling relationship between buoyancy force Fb,Marangoni force F? and indirect light pressure Fi was found by force analysis of bubbles in the nanofluid.The position of the bubble centroid relative to the laser beam will affect the Marangoni force and indirect light pressure.The Marangoni force and indirect light pressure decrease when the center of the bubble is far away from the laser beam.The balance between buoyancy and vertical Marangoni forces F?,y determines the position of the bubble's center of mass in the vertical direction,which affect the balance between indirect light pressure and horizontal Marangoni forces F?,x.The temperature gradient generating Marangoni force in the liquid and the indirect light pressure were changed by adjusting the parameters of the pulse laser,so that the bubbles presented different flow patterns.Using pulsed laser to illuminate nano fluid can effectively hold back chaotic movement of bubbles and regulate flow patterns of bubbles in the liquid.The flow patterns of bubble discussed in this paper have potential applications in microreactors,microfluidic chips,drug delivery and signal processing,providing ideas for the application of light-controlled bubble motion and guiding researchers to further explore the process and mechanism of light-controlled bubble motion.