^{1,a)}and M. Kakehata

^{1}

### Abstract

We proposed and experimentally demonstrated a novel method of evaluating the number density of droplets in an aerosol by laser-induced breakdown. The number density of droplets is evaluated from the volume in which the laser intensity exceeds the breakdown threshold intensity for droplets, and the number of droplets in this volume, which is evaluated by the experimentally observed breakdown probability. This measurement method requires a large number of laser shots for not only precise measurement but also highly temporally and spatially resolved density distribution in aerosol. Laser ablation plumes ejected from liquid droplets generated by breakdown disturb the density around the measurement points. Therefore, the recovery time of the density determines the maximum repetition rate of the probe laser irradiating a fixed point. The expansion range of the ablation plume determines the minimum distance at which the measurement points are unaffected by a neighboring breakdown when multiple laser beams are simultaneously irradiated. These laser irradiation procedures enable the measurement of the number density distribution of droplets in an aerosol at a large number of points within a short measurement time.

The authors thank Dr. Kenji Torizuka for their advice and support during this study.

I. INTRODUCTION

II. EXPERIMENTAL PROCEDURE

III. EXPERIMENTAL RESULTS

A. Experiment 1: Temporal variation of droplet density for high-repetition laser irradiation

B. Experiment 2: Expansion range of ablation plume for multiple laser irradiations

IV. DISCUSSION

V. SUMMARY

### Key Topics

- Laser ablation
- 63.0
- Fluid drops
- 49.0
- Aerosols
- 46.0
- Photon density
- 23.0
- Laser induced breakdown
- 20.0

## Figures

Schematic of experimental setup. Exit of the spray nozzle is set at z = 100 mm. Aerosol is ejected downward and spread out at an angle to vertical direction.

Schematic of experimental setup. Exit of the spray nozzle is set at z = 100 mm. Aerosol is ejected downward and spread out at an angle to vertical direction.

The breakdown probabilities of the probe laser and the number density of droplets as a function of the delay time between the pump laser and probe laser. The pump and probe lasers are focused on the same point at the coordinate origin with an interval given by the delay time. The intensities of the pump and probe lasers are 9.2 × 1010 and 1.1 × 1010 [W/cm2] for (a) and 1.5 × 1011 and 2.2 × 1010 [W/cm2] for (b), respectively. The aerosol is ejected at a water flow rate of 10 ml/min and an air pressure of 0.4 MPa at the nozzle for (a), and at a water flow rate of 20 ml/min and an air pressure of 0.1 MPa at the nozzle for (b).

The breakdown probabilities of the probe laser and the number density of droplets as a function of the delay time between the pump laser and probe laser. The pump and probe lasers are focused on the same point at the coordinate origin with an interval given by the delay time. The intensities of the pump and probe lasers are 9.2 × 1010 and 1.1 × 1010 [W/cm2] for (a) and 1.5 × 1011 and 2.2 × 1010 [W/cm2] for (b), respectively. The aerosol is ejected at a water flow rate of 10 ml/min and an air pressure of 0.4 MPa at the nozzle for (a), and at a water flow rate of 20 ml/min and an air pressure of 0.1 MPa at the nozzle for (b).

The breakdown probabilities of the probe laser as a function of the delay time for different vertical positions of the breakdown caused by the pump laser. The pump laser is focused at z = 4.0, 3.0, 2.0, 1.0, 0, −0.5, and −1.0 mm for (a), (b), (c), (d), (e), (f), and (g), respectively. Fig. 3(e) is the same result as shown in Fig. 2(a) . The other experimental parameters are thesame as those in Fig. 2(a) . Scales for white legend symbols in (a), (c), (e), and (g) are left hand side and for black ones in (b), (d), and (f) are right hand side, respectively.

The breakdown probabilities of the probe laser as a function of the delay time for different vertical positions of the breakdown caused by the pump laser. The pump laser is focused at z = 4.0, 3.0, 2.0, 1.0, 0, −0.5, and −1.0 mm for (a), (b), (c), (d), (e), (f), and (g), respectively. Fig. 3(e) is the same result as shown in Fig. 2(a) . The other experimental parameters are thesame as those in Fig. 2(a) . Scales for white legend symbols in (a), (c), (e), and (g) are left hand side and for black ones in (b), (d), and (f) are right hand side, respectively.

The breakdown probabilities of the probe laser as a function of delay time for different horizontal positions of the breakdown caused by the pump laser. The pump laser is focused at x = 2.0, 1.0, and 0 mm for (a), (b), and (c), respectively. Fig. 4(c) is the same result as shown in Fig. 2(a) . The other experimental parameters are the same as those in Fig. 2(a) .

The breakdown probabilities of the probe laser as a function of delay time for different horizontal positions of the breakdown caused by the pump laser. The pump laser is focused at x = 2.0, 1.0, and 0 mm for (a), (b), and (c), respectively. Fig. 4(c) is the same result as shown in Fig. 2(a) . The other experimental parameters are the same as those in Fig. 2(a) .

The pump laser focus positions, where its breakdown affects the other breakdown at coordinate origin, are shown as black circles. Other positions, where its breakdown does not affect, are shown as white circles. These points were obtained from the experimental results shown in Figs. 2 – 4 .

Relationship between the pump laser focus position and the delay time corresponding to the initial decrease in breakdown probability t f, the minimum breakdown probability t b, and the full recovery of the breakdown probability tr as shown in reference diagram, where Pb is the breakdown probability. The solid line is the fitting line for t b. The dashed lines have the same gradient as the solid line, enabling a comparison of the relationships between the distance between the position of the pump laser and the delay times t f and t r.

Relationship between the pump laser focus position and the delay time corresponding to the initial decrease in breakdown probability t f, the minimum breakdown probability t b, and the full recovery of the breakdown probability tr as shown in reference diagram, where Pb is the breakdown probability. The solid line is the fitting line for t b. The dashed lines have the same gradient as the solid line, enabling a comparison of the relationships between the distance between the position of the pump laser and the delay times t f and t r.

Distances of plume expansion as a function of expansion time, shown as black points for z-axis and with point for x-axis with experimental errors. These points were obtained from the results shown in Figs. 3 and 4 . The expansion distances take into consideration the aerosol flow velocity and the direction as a vector summation. The dashed line is a best-fitting curve obtained using a drag force model.

Distances of plume expansion as a function of expansion time, shown as black points for z-axis and with point for x-axis with experimental errors. These points were obtained from the results shown in Figs. 3 and 4 . The expansion distances take into consideration the aerosol flow velocity and the direction as a vector summation. The dashed line is a best-fitting curve obtained using a drag force model.

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