| Author: | Gao, Lihao |
| Title: | Dynamics of drops and microjets subjected to intense laser blasts |
| Advisors: | Tang, Hui (ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Drops -- Effect of lasers on Jets -- Effect of lasers on X-ray lasers Laser ablation Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xli, 175 pages : color illustrations |
| Language: | English |
| Abstract: | The interaction between laser and liquid objects (drops, jets, films) has drawn much attention in the past few decades. This topic is of fundamental importance in both industrial and scientific fields, such as the inertial confinement fusion, the extreme ultraviolet source for nanolithography, and the serial femtosecond crystallography technology based on the X-ray free electron laser. The pulsed laser blast deposits significant energy to the liquid object on a tight spot in a few nanoseconds or less, resulting in an extremely high power flux. Such interactions involve complex processes, including strong localized heating, ionization, phase change, shockwave emission and propagation, matter ejection, and light transmission. Previous studies have primarily focused on the drop deformation and fragmentation driven by the laser blast and the shockwave propagation within the liquid jet. However, there is still much research needed for establishing a more complete physical picture of the laser blast process. In this work, we designed and built an automatic time-resolved high-speed photography system to visualize the laser blast at an effective frame rate up to 20,000,000 frames per second and an effective exposure time as short as 5 ns. A multi-function optical path system was built to transmit, control and measure each high energy laser pulse. The light source, liquid object generation device and flow visualization methods were designed. We added fluorescence dye into the liquid for direct observation of the evolution within the liquid object, with which some phenomena were discovered for the first time. The major findings are as follows. Firstly, we visualized the laser blast of transparent drops and characterized the phenomena including formation of cavitation bubbles, emission of shockwave, violent rupture of the drop surface, and ejection of matter. Four energy-density-dependent fragmentation regimes were identified: the fine atomization, the formation and evolution of unstable or stable liquid sheet, and the coarse fragmentation. Two types of bubbles were observed after the laser blast: dense nanobubbles that populate the drop surface and sparse isolated micro bubbles. The shockwave propagation velocity within the drop is ~1450 m/s, comparable to sound speed in the liquid. The energy released in each shockwave emission event after drop surface rupture is estimated to be ~10% of the laser pulse energy. The diameter of the ejected microdroplets is estimated to be ~120 nm. The light path within the drops was simulated to study the effects of numerical aperture, drop location and drop spheroid deformation on light transmission within the drop. The results show that the spherical aberration of the lens is augmented by the drop refocus, leading to the formation of multiple focal points with laser beam of large numerical aperture (NA ≥ 0.24). The diameter of laser spot has minor influence on the light path while both numerical aperture and the drop location relative to the lens have major impact on the light path. The spheroid deformation leads to the disappear of single focal point while more diverged beam paths appear. Secondly, we studied the dynamics of a millimeter-sized gallium-indium drop ablated by intense laser pulses. This physical process involves four timescales: the laser pulse, the duration of plasma, the inertial timescale and the capillary timescale. The energy released during the blast is 60%~98% of the laser pulse energy, according to the shockwave propagation and Taylor's blast model. The peak pressure and temperature behind the shockwave are as high as 30 Bar and 2000 K within 500 ns after the blast, respectively. The drop deformation is described by the laser energy and time dependent scaling laws. Through this study, the application range of a power law that describes the relation between the laser pulse energy and the lateral drop motion derived from the micrometer-sized indium-tin drop was extended to the millimeter-sized gallium-indium drop. Thirdly, we studied the response of the ~100 micron water jets to intense nanosecond laser blasts. The blast center, the nozzle orifice and the breakup point of the jet were all observed. We identified four timescales of this process, including the laser pulse duration, ejection of matter, the inertial timescale and the capillary timescale. The diameter of the ejected droplets is ~100 nm. The gap that breaks the liquid column near the blast center expands logarithmically. A liquid sheet appears at the end of each liquid column and experiences the temporal evolution of three stages. When the jet is transparent to laser, another liquid sheet forms near the nozzle orifice. The nozzle tip damage could be observed after a few blasts of transparent jets, and the damage pattern is influenced by the meniscus curvature at the nozzle orifice. The laser blast leads to the formation of a Christmas-tree-like pattern at the breakup point of the transparent jet. We found that less than 0.006% of optical power could be coupled into the jet by whispering-gallery-mode when the liquid jet was illuminated by a continuous wave laser, but about 3.5% of the optical energy can be coupled into the jet by the plasma reflection upon laser blast, and a half of the coupled light will be transmitted upstream through a long-range as far as 50 times of jet diameter to the nozzle orifice. We further revealed that only the primary laser irradiation, which was about a half of the optical energy arrived the nozzle orifice, reached the damage threshold of the nozzle tip material. Temperature rise of the liquid near the interface between the nozzle material and the liquid was above the superheating point of the liquid and thus can lead to localized boiling. Laser blast dynamics of three different liquid objects (transparent drop, metal drop, jet) were compared from the aspects of timescales, matter ejection and shockwave emission. The comparison shows that both the temporal evolution process and the energy deposition rate are influenced by the liquid properties. The instant laser energy deposition is a localized process defined by the optical path and boundary, but the subsequent evolution is a global process that is influenced by the characteristic length of the liquid object. This research has enriched our understanding of the interaction between laser and liquid objects, providing useful information for the future industrial and scientific applications. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/11927