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Abstract
Bubbles can be formed by focusing a high-power laser in a liquid. Based on this phenomenon, the present study demonstrated a novel technique, referred to as microFabrication using Laser-Induced Bubbles (microFLIB), for the microfabrication of the thermoset polymer polydimethylsiloxane (PDMS). A conventional nanosecond green laser was focused at the interface between uncured PDMS and a metal target and scanned to generate a line of bubbles at the boundary. The hemispherical shapes of these bubbles produced a groove on the rear side of the PDMS substrate following subsequent thermal curing. After the fabrication of such specimens, metal films could be selectively deposited along the grooves by electroless plating. This process allows rapid, high-quality microfluidic fabrication with potential applications to biochips.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Polydimethylsiloxane (PDMS) is a versatile material that has been widely used in the fields of biology [1,2], medicine [3–6] and tissue engineering [7,8]. This polymer has numerous useful characteristics, including a high degree of transparency, resistance to various chemicals, low density and minimal cost [9]. For these reasons, PDMS has been employed in the construction of microfluidic devices since Whitesides et al. first used PDMS as a microfluidic chip material in 1998 [10]. A soft-lithography technique originally developed by the semiconductor industry has been employed to pattern the surfaces of polymer substrates in order to fabricate complex microfluidic devices made of PDMS. However, this process involves multiple steps [11] and requires access to advanced clean room facilities similar to those used for microelectronics, which are typically costly. The direct laser writing of PDMS using conventional nanosecond lasers via single-photon absorption is an attractive alternative method that allows rapid prototyping [12,13]. Furthermore, both laser-induced plasma-assisted ablation [14] and laser-induced backside wet etching [15] enable the microfabrication of transparent polymer materials via a single step, by positioning a laser-absorbing medium such as a metal target or an organic solution behind the polymer. As a result, the rear side of the substrate can be microfabricated even if the wavelength of the laser beam is not absorbed by the polymer. These medium-mediated fabrication techniques can also selectively deposit thin metal films on the processed areas [16] or provide deep etching without significant roughness [17]. However, the majority of conventional laser processing techniques, including the medium-mediated techniques described above, allow only low fabrication rates (typically less than a few μm/pulse [12–17]) due to the use of solid transparent polymers as substrates.
Laser ablation in liquid (LAL) has been utilized in many applications, because it permits the high-quality cleaning and peening of substrate surfaces [18] as well as the synthesis of nanoparticles [19–21]. The laser-induced formation of bubbles during the LAL process has been well-documented by many groups, and has been found to affect the fabrication process, such as by preventing the incoming laser pulse. Wisan et al. investigated the dynamic behavior of bubbles generated by multiple laser pulses [22], while Tanabe et al. studied the dynamics of such bubbles using high-speed laser stroboscopic videography [23]. Mahdieh et al. also examined both bubble formation and ablation rates during LAL experiments using an optical transmission technique [24]. These past studies provided an improved understanding of the bubble formation characteristics, including the growth, shrinkage and collapse that occur during LAL, and suggest the viability of efficient laser processing in liquid environments.
Our own group has developed a novel technique for the microfabrication of PDMS using laser-induced bubbles, termed microFLIB (for “microFabrication using Laser-Induced Bubbles”). In this process, the interface between uncured liquid PDMS and a metal target is irradiated using a conventional pulsed green laser. This technique has been shown to permit rapid, high-quality microfluidic fabrication of structures on the PDMS surface, and has some advantages over standard microfluidic fabrication methods. In the work reported herein, various fundamental characteristics of the microFLIB processing of PDMS were investigated, and the associated mechanism was elucidated. Additionally, selective metallization of the processed areas was performed by successive electroless plating. Following the development of the fabrication technique, a three dimensional (3D) microfluidic chip was created by simply bonding the PDMS substrate fabricated by microFLIB to a cover glass, and liquid flow in the fabricated microfluidic chip was demonstrated.
2. Experiments
2.1. microFLIB
A commercially available uncured PDMS and a curing agent (Silpot 184 W/C), purchased from the Toray Dow Corning Co., Ltd., were mixed at a mass ratio of 10:1. These materials were stirred for 30 min in a beaker and poured into a glass container in which the copper (Cu) target was positioned at the bottom. Figure 1(a) presents a schematic illustration of the experimental setup. The irradiation source was a conventional pulsed green laser (Nd:YAG /Cr:YAG microchip laser: 532 nm, 0.5 ns, 1 kHz), with the output power adjusted by neutral density (ND) filters. Plano-convex and plano-concave lenses were positioned for the expansion of the collimated laser beam diameter to fill the entrance pupil of the objective lens ( × 20, numerical aperture (NA) = 0.35). The laser beam was focused at the interface between the uncured PDMS and the Cu target (Having regard to the fact that the size of the generated bubble is bigger than that of the focused laser beam described later, the laser beam was simply focused at the interface even though the focal position and the spot size of the focused laser beam given by the NA of the objective lens are altered due to the refractive index mismatch between air and uncured PDSM (n = 1.41).). During irradiation by the laser, the entire container was moved using a computer-controlled xyz stage.
Fig. 1 Schematic illustrations of the microFLIB process. (a) The experimental setup and (b) enlarged images showing the Cu/uncured PDMS boundary during laser irradiation. The focused laser beam was scanned parallel to the Cu surface to generate a line of bubbles.
As shown in Fig. 1(b), bubbles were generated along the line of laser scanning at the Cu/uncured PDMS interface. Subsequently, the entire container was placed in a programmable furnace and heated at 80 °C for 90 min to cure the PDMS. Following the thermal cure, the PDMS substrate on the rear side of the Cu target, containing the groove produced by the microFLIB process, was peeled off. In addition to the surface microfabrication of the PDMS substrate, selective metallization of the fabricated areas on the rear side of the substrate was attempted. This was accomplished by immersing the fabricated substrate in an electroless Cu plating solution (C-200LT, Kojundo Chemical Laboratory Co., Ltd.) for 20 min at 50 °C. All experiments were repeated five times. The surface characteristics of the PDMS substrate following the microFLIB treatment were analyzed using laser scanning microscopy (LSM) and transmission microscopy (TM), while energy dispersive X-ray spectrometry (EDX) was used to assess the selective metallization.
3. Results and discussion
3.1. Fundamental characteristics of microFLIB process
As noted, a focused laser beam was passed through the uncured PDMS and scanned over the Cu target to generate bubbles along the scanning line at the Cu/uncured PDMS boundary (see Visualization 1). Initially, the laser power and the scanning speed were kept constant at 10 mW and 10 mm/sec, respectively (The scanning speed of 10 mm/s was selected to be the maximum speed of the motorized stage). Figure 2 shows microscopy images of the line of bubbles formed on the Cu target as viewed from the top and side. Following the laser irradiation, the bubbles were observed to remain at the interface for more than 3 h while retaining their hemispherical cross-sectional shape, after which they collapsed from the edge. Figure 2(c) and 2(d) provide a photographic and a 3D LSM images of the rear side of the PDMS substrate after it was peeled away from the Cu target following thermal curing, while Fig. 2(e) shows a cross-sectional view of Fig. 2(d). It is evident that microgrooves were fabricated on the rear side of the PDMS substrate where the lines of bubbles were formed in the uncured PDMS. The aspect ratio of the each groove was determined to be approximately 1. Diameters of the focused laser beam and the generated bubbles were approximately 20 and 30 μm respectively, measured from the ablated hole of the Cu target and the width of the fabricated microgroove. Following the ablation, the bubble expands uniformly maintaining its hemispherical shape over the ablated hole on the Cu target as well as other bubble generation described in Ref [22–25]. Thus, rapid microgroove fabrication in PDMS with a high aspect ratio was achieved using only a single laser scan. This level of performance would be difficult to achieve by using the conventional laser processing techniques described in the Introduction.
Fig. 2 Microscopy images of laser-induced bubbles at the Cu/uncured PDMS boundary after the microFLIB process, taken from the (a) top and (b) (Visualization 1) side of the specimen. The generated bubbles remain at the boundary for more than 3 h and then collapse from the edge. (c) A photographic image of the lines of microgrooves fabricated on the rear side of the PDMS. (d) An enlarged LSM image of the groove and (e) the associated cross-sectional profile. The microgroove with aspect ratio of approximately 1 can be obtained by the microFLIB of single laser scanning.
A wavy shape at the bottom of the microgroove was observed following the use of a decreased laser scanning speed during the microFLIB processing. Figure 3 shows a 3D LSM image of a microgroove fabricated with the same laser power used to produce the groove in Fig. 2, but at a scan rate of 5000 μm/s. Figure 3(b) presents the cross-sectional profile measured at the center of the fabricated microgroove, as indicated by the dashed line in Fig. 3(a), while Fig. 3(c) shows the same profile but for the specimen in Fig. 2(c).
Fig. 3 (a) LSM image of the microgroove fabricated using a low laser scanning speed, and (b) and (c) cross-sectional profiles obtained from (a) and Fig. 2(c), respectively. A microgroove with a wavy shape at the groove base is fabricated using a low scanning speed, while a smooth surface at the base is fabricated using a higher scanning speed.
It is evident that the base of the microgroove produced at 5000 μm/s corresponds to the wavy shape of the upper part of the laser-induced bubbles. These results demonstrate that the appropriate processing conditions must be applied to obtain a smooth surface at the bottom of the microgroove. Subsequently, the optimal laser irradiation conditions were assessed by varying both the laser power and the scanning speed to ensure high-quality microgroove fabrication.
The degree of waviness at the base of each microgroove obtained with various laser irradiation conditions was determined, using LSM. A smooth surface was defined as that for which the arithmetic mean of the modulations along the base was less than 1 μm, and the results are summarized in Table 1. These data show that fabricating a smooth microgroove using the microFLIB process requires a relatively fast scanning speed as the laser power is increased, and vice versa. The reason why the wavy shape at the groove base formed by the slow scanning speed is unclear due to the interactions such as the irradiation of the following refracted laser beam to the preformed bubble and internal pressure dynamics of the two bubbles during their connection. Thus, a suitable degree of overlap of the bubbles being generated is evidently necessary for high quality microgroove fabrication.
Table 1. Effects of laser irradiation conditions on the waviness at the base of the fabricated microgroove. ○: Arithmetic mean is less than 1 μm. × : Arithmetic mean is more than 1 μm.
Figure 4(a) plots the surface roughness at the base of the microgroove as a function of the overlap ratio of the focused laser beam at various laser powers. In these trials, the overlap ratio was calculated from the diameters of the ablated hole on the Cu target and the laser scanning speed was optimized to maintain a constant overlap when changing the power. It is apparent that the surface roughness is significantly improved at an overlap ratio of approximately 50%. At this overlap, the upper parts of the laser-generated bubbles are suitably connected, resulting in a smooth surface at the bottom of the microgroove. Using this overlap ratio, the effect of the laser power on the size of the microgroove was assessed, as shown in Fig. 4(b). These data indicate that the size of the fabricated microgroove (that is, both the width and the depth) increases as the power is increased. This effect is attributed to the formation of larger bubbles.
Fig. 4 (a) Surface roughness at the microgroove base as a function of overlap ratio at various laser powers, and (b) microgroove dimensions as functions of the laser power. A smooth microgroove with hemispherical shape can be obtained using an appropriate overlap ratio of the laser beam during the microFLIB. The size of the microgroove can be controlled by varying the laser power.
3.2. Selective metallization
The results described in Sections 3.1 demonstrated that microgrooves could be fabricated on the PDMS substrate by forming laser-induced bubbles at the metal/uncured PDMS boundary followed by thermal curing. Subsequent to this process, selective metallization of the microgroove was attempted by means of electroless plating. In this process, the patterned PDMS substrate was immersed in an electroless Cu plating solution. Figure 5 shows microscopy images of the groove fabricated on the PDMS substrate before and after the plating, together with enlarged and cross-sectional images.
Fig. 5 Microscopy images of the fabricated microgroove (a) before and (b) after the electroless Cu plating, and (c) an enlarged image of the area indicated by the circle in (b) after the tape test. (d) A cross-sectional image obtained from (c). Cu thin film is selectively deposited on the fabricated microgroove after the plating.
It is evident from these images that a thin Cu film with a thickness of approximately 2.3 μm was selectively deposited on the microgroove after the electroless plating as shown in Fig. 5(b) – 5(d). The linewidth of the Cu film in Fig. 5(b) and 5(c) almost agrees with that of fabricated microgroove in Fig. 5(a) since the Cu film mainly deposits on the surface of the microgroove as shown in the cross-sectional image of the plated groove in Fig. 5(d). In addition, the Cu film remained attached on the fabricated groove, while the film slightly deposited over the linewidth of the groove shown in Fig. 5(b) was easily peeled off after the Scotch tape test to assess the adhesion as presented in Fig. 5(c). The resistance of the plated groove was measured to be 14 Ω, which indicates that the groove is electrically conductive.
During the microFLIB process, the metal target undergoes laser ablation at the metal/uncured PDMS interface. This, in turn, generates a plasma plume that includes metal particles within the bubble generated by the laser irradiation. After curing the PDMS, the metal particles evidently remain on the polymer and act as seeds for the plating process, such that plating proceeds solely at the microgroove. The selective deposition of metal on the microgroove was further assessed by a qualitative mapping analysis using EDX to generate the elemental mapping images in Fig. 6. These maps were acquired after the microgroove fabrication, after treatment of the fabricated groove using hydrochloric acid in an ultrasonic bath for 5 min and after the electroless Cu plating (without the hydrochloric acid treatment). The upper row of figures presents microscopy images of the regions examined by EDX. In the elemental maps, Cu and Si appear as red and green, respectively.
Fig. 6 Microscopy images (upper) and enlarged elemental maps (lower) of a microgroove after (a) the microFLIB process, (b) hydrochloric acid treatment and (c) electroless plating without the hydrochloric acid treatment. In the EDX maps, Cu and Si show as red and green, respectively.
These maps demonstrate the presence of some Cu particles ablated from the target on the fabricated microgroove, and also show that some of these particles were removed by the hydrochloric acid treatment. A rough quantitative analysis based on the EDX data showed that, after a five min ultrasonic cleaning, the Cu concentration in a 20 × 20 μm area at the bottom of the microgroove was reduced from 2.8 to 0.9 wt%. It is also apparent that selective metallization was achieved on the microgroove after the electroless plating when the fabricated microgroove was placed in the plating solution without the acid treatment. Therefore, selectively depositing a metal film on the fabricated areas requires a metal target to be ablated during the microFLIB process.
3.3. MicroFLIB mechanism
During the microFLIB processing of uncured PDMS, the bubbles that were produced on the Cu target were retained for more than 3 h, as shown in Fig. 2. There have been many dynamic analyses of the growth, shrinkage and collapse of laser-induced bubbles formed by a single nanosecond laser ablation at metal/water interfaces. Previous work has demonstrated that laser-induced bubbles in water will eventually collapse on the μs time scale [22–25]. However, in the present trials, the bubbles remained intact for longer time spans.
We attempted to clarify the mechanism by which the bubbles generated at the Cu target were retained. Figure 7 shows time-lapse images of bubbles at the Cu/uncured PDMS interface resulting from a single laser pulse and from ten pulses, using the same laser power as employed to generate the specimens shown in Fig. 2 and maintaining the same irradiation position. Figure 7(a) shows a floating bubble formed by a single laser pulse 120 s after irradiation. A narrow, membrane-like structure is seen in the wake of the spherical bubble, as indicated by the red dashed circles. This membrane-like structure is also clearly evident following the application of ten pulses, as in Fig. 7(b). In this case, following the bubble formation, the membrane-like structure appears to connect the surface of the Cu target and the lower part of the bubble. At the 420 s point after bubble formation, the bubble starts to float upwards, leaving the membrane in its wake. The bubble was retained on the Cu target for a longer than in Fig. 7(a), likely due to the larger amount of membrane that was formed. Based on these results, we propose that the longer retention time of the bubbles at the Cu/uncured PDMS interface results from the mechanism shown schematically in Fig. 8.
Fig. 7 Time-lapse images of the laser-induced bubbles formed by (a) a single pulse and (b) ten pulses of the nanosecond laser. Following the multiple laser irradiations, membrane-like structure was clearly observed at the Cu/lower part of the bubble.
Fig. 8 Schematic illustration of the microFLIB mechanism. Following laser irradiation, membrane-like structures are formed at the metal/bubble boundary (Shaded areas represent the membrane-like structures.).
In this process, the nanosecond laser ablation of the Cu/uncured PDMS boundary produces a high temperature plasma (greater than several thousand degrees K) within several nanoseconds (leftmost image) [26,27]. During the expansion and extinction of this plasma, a thin membrane-like structure made from thermally cross-linked PDMS is generated around the plasma (center image). Following the plasma extinction, a laser-induced shock wave propagates, forming a bubble that may move upward and expand the membrane (rightmost image). As a result, the membrane structure covers and retains the laser-induced bubble at the Cu target for a longer time span compared to previous studies of bubbles created on metal/water interfaces. Furthermore, when a line of bubbles is formed by overlapping the laser pulses, the amount of the membrane increases and the membranes may link to one another, resulting in even stronger retention of the bubbles. The exact details of this mechanism are still unclear, but it appears certain that the membranes generated around the bubble play a role in bubble stabilization.
3.4. Microfluidic chip fabrication using microFLIB
A microfluidic channel was fabricated on the PDMS substrate by microFLIB after which, by simply bonding both the PDMS substrate to a cover glass, a 3D microfluidic chip was fabricated. Figure 9(a) shows a schematic illustration of the microfluidic chip, comprising two mechanically punched reservoirs that are both connected to the microfluidic channel, while Fig. 9(b) shows a photographic image of the fabricated microfluidic chip. An enlarged microscopy image of the microfluidic channel is presented in Fig. 9(c). During the microFLIB process, a single scan of the ns laser beam was employed to form a line of bubbles so as to produce the channel. Following the microfluidic chip fabrication, distilled water containing blue ink was injected from one of the reservoirs to demonstrate flow through the channel, as shown in the microscopy image in Fig. 9(d). Liquid was confirmed to flow through the channel just as occurs in conventional microfluidic chips. Thus, this newly developed microFLIB technique may permit high-speed fabrication of high-quality microfluidic channels.
Fig. 9 (a) Schematic illustration and (b) a photographic image of the microfluidic chip. Enlarged microscopy images of (c) the microfluidic channel fabricated by microFLIB and (d) the same microfluidic channel filled with water containing blue ink.
4. Summary
This work developed a novel technique for the microfabrication of the thermoset polymer PDMS, using laser-induced bubbles, referred to as microFLIB. In this process, a conventional nanosecond laser pulse is focused and scanned at the metal/uncured PDMS boundary to generate a line of bubbles. Following the thermal cure of the PDMS containing the bubbles, a microgroove with a hemispherical shape is obtained at the rear side of the PDMS substrate. This work demonstrated that high-quality microfabrication can be achieved by adjusting the overlap of the focused laser beam. Furthermore, selective metallization of the fabricated microgroove can be realized by successive electroless plating. An study of the associated mechanism determined that membrane-like structures generated by thermal cross-linking formed around the bubbles so as to retain the bubbles at the boundary after the microFLIB.
High-speed microfluidic fabrication was demonstrated using the microFLIB technique. A channel produced by this method enabled liquid injection just as can be seen in other conventional microfluidic chips. Thus, microFLIB permits the high-speed and high-quality microfabrication of PDMS and should be applicable not only to PDMS microfluidics but also other industrial uses of thermoset materials.
References
1. K. J. Regehr, M. Domenech, J. T. Koepsel, K. C. Carver, S. J. Ellison-Zelski, W. L. Murphy, L. A. Schuler, E. T. Alarid, and D. J. Beebe, “Biological implications of polydimethylsiloxane-based microfluidic cell culture,” Lab Chip 9(15), 2132–2139 (2009). [CrossRef] [PubMed]
2. H. Zhang and M. Chiao, “Anti-fouling coatings of Poly(dimethylsiloxane) devices for biological and biomedical applications,” J. Med. Biol. Eng. 35(2), 143–155 (2015). [CrossRef] [PubMed]
3. S. H. Kim, J. H. Moon, J. H. Kim, S. M. Jeong, and S. H. Lee, “Flexible, stretchable and implantable PDMS encapsulated cable for implantable medical device,” Biomed. Eng. Lett. 1(3), 199–203 (2011). [CrossRef]
4. J. Mokkaphan, W. Banlunara, T. Palaga, P. Sombuntham, and S. Wanichwecharungruang, “Silicone Surface with Drug Nanodepots for Medical Devices,” ACS Appl. Mater. Interfaces 6(22), 20188–20196 (2014). [CrossRef] [PubMed]
5. J. Lu and T. M. Kowalewski, “Flexible, stretchable skin sensors for two-dimensional position tracking in medical simulators,” ASME J. Med. Devices 9, 020927 (2015).
6. J. Chen, J. Zheng, Q. Gao, J. Zhang, J. Zhang, O. M. Omisore, L. Wang, and H. Li, “Polydimethylsiloxane (PDMS)-based flexible resistive strain sensors for wearable applications,” Appl. Sci. (Basel) 8(3), 345–360 (2018). [CrossRef]
7. E. Pedraza, A. C. Brady, C. A. Fraker, and C. L. Stabler, “Synthesis of macroporous poly(dimethylsiloxane) scaffolds for tissue engineering applications,” J. Biomater. Sci. Polym. Ed. 24(9), 1041–1056 (2013). [CrossRef] [PubMed]
8. A. Khademhosseini, R. Langer, J. Borenstein, and J. P. Vacanti, “Microscale technologies for tissue engineering and biology,” Proc. Natl. Acad. Sci. U.S.A. 103(8), 2480–2487 (2006). [CrossRef] [PubMed]
9. S. S. Saliterman, FUNDAMENTALS OF BioMEMS and Medical Microdevices: Silicon Microfabrication, 2.2 Lithography (SPIE, 2006).
10. E. K. Sackmann, A. L. Fulton, and D. J. Beebe, “The present and future role of microfluidics in biomedical research,” Nature 507(7491), 181–189 (2014). [CrossRef] [PubMed]
11. Y. Xia and G. M. Whitesides, “Soft Lithography,” Angew. Chem. Int. Ed. Engl. 37(5), 550–575 (1998). [CrossRef] [PubMed]
12. K. Liu, Z. NiCkolov, J. Oh, and H. Moses Noh, “KrF excimer laser micromachining of MEMS materials: characterization and applications,” J. Micromech. Microeng. 22, 015012 (2012). [CrossRef]
13. Y. K. Hsieh, S. C. Chen, W. L. Huang, K. P. Hsu, K. A. V. Gorday, T. Wang, and J. Wang, “Direct micromachining of microfluidic channels on biodegradable materials using laser ablation,” Polymers (Basel) 9(12), 242 (2017). [CrossRef]
14. Y. Hanada, K. Sugioka, H. Takase, H. Takai, I. Miyamoto, and K. Midorikawa, “Selective metallization of polyimide by laser-induced plasma-assisted ablation (LIPAA),” Appl. Phys., A Mater. Sci. Process. 80(1), 111–115 (2005). [CrossRef]
15. J. Wang, H. Niino, and A. Yabe, “Microfabrication of a fluoropolymer film using conventional XeCl excimer laser by laser-induced backside wet etching,” Jpn. J. Appl. Phys. 38(Part 2, No. 7A), L761–L763 (1999). [CrossRef]
16. Y. Hanada, K. Sugioka, Y. Gomi, H. Yamaoka, O. Otsuki, I. Miyamoto, and K. Midorikawa, “Development of practical system for laser-induced plasma-assisted ablation (LIPAA) for micromachining of glass materials,” Appl. Phys., A Mater. Sci. Process. 79(4-6), 1001–1003 (2004). [CrossRef]
17. Y. Kawaguchi, T. Sato, A. Narazaki, R. Kurosaki, and H. Niino, “Rapid prototyping of silica glass microstructures by the LIBWE method: Fabrication of deep microtrenches,” J. Photochem. Photobiol. Chem. 182(3), 319–324 (2006). [CrossRef]
18. A. Kruusing, “Underwater and water-assisted laser processing: Part 1—general features, steam cleaning and shock processing,” Opt. Lasers Eng. 41(2), 307–327 (2004). [CrossRef]
19. M. Ganjali, M. Ganjali, P. Vahdatkhah, and S. M. B. Marashi, “Synthesis of Ni nanoparticles by pulsed laser ablation method in liquid phase,” Procedia Materials Science 11, 359–363 (2015). [CrossRef]
20. V. Amendola and M. Meneghetti, “Laser ablation synthesis in solution and size manipulation of noble metal nanoparticles,” Phys. Chem. Chem. Phys. 11(20), 3805–3821 (2009). [CrossRef] [PubMed]
21. T. B. Nguyen, T. D. Nguyen, Q. D. Nguyen, and T. T. Nguyen, “Preparation of platinum nanoparticles in liquids by laser ablation method,” Adv. Nat. Sci.: Nanosci. Nanotechnol. 5(3), 035011 (2014). [CrossRef]
22. W. Charee and V. Tangwarodomnukun, “Dynamic features of bubble induced by a nanosecond pulse laser in still and flowing water,” Opt. Laser Technol. 100, 230–243 (2018). [CrossRef]
23. R. Tanabe, T. T. P. Nguyen, T. Sugiura, and Y. Ito, “Bubble dynamics in metal nanoparticle formation by laser ablation in liquid studied through high-speed laser stroboscopic videography,” Appl. Surf. Sci. 351, 327–331 (2015). [CrossRef]
24. M. H. Mahdieh and M. Akbari Jafarabadi, “Bubble formation induced by nanosecond laser ablation in water and its diagnosis by optical transmission technique,” Appl. Phys., A Mater. Sci. Process. 116(3), 1211–1220 (2014). [CrossRef]
25. A. Matsumoto, A. Tamura, A. Kawasaki, T. Honda, P. Gregorcic, N. Nishi, K. Amano, K. Fukami, and T. Sakka, “Comparison of the overall temporal behavior of the bubbles produced by short- and long-pulse nanosecond laser ablations in water using a laser-beam-transmission probe,” Appl. Phys., A Mater. Sci. Process. 122(3), 234 (2016). [CrossRef]
26. K. Sasaki and N. Takada, “Liquid-phase laser ablation,” Pure Appl. Chem. 82(6), 1317–1327 (2010). [CrossRef]
27. C. Y. Shih, C. Wu, M. V. Shugaev, and L. V. Zhigilei, “Atomistic modeling of nanoparticle generation in short pulse laser ablation of thin metal films in water,” J. Colloid Interface Sci. 489, 3–17 (2017). [CrossRef] [PubMed]
