The experimental setup we used (Fig.
1) is similar to that used in previous contributions (Tagawa et al.
2012,
2013; Hayasaka et al.
2017). A glass tube (inner diameter of 500 µm) was filled with magenta ink (THC7M4N, ELECOM Co.) and irradiated by a Nd:YAG pulsed laser (Nano S PIV, Litron Lasers Co., wavelength 532 nm, pulse duration 6 ns, the size of the laser focal spot 2.6 µm (see Tagawa et al.
2016)), where the laser energy ranged within 0.6–3.0 mJ. The focused liquid jet emerged from the curved meniscus and then impacted and penetrated the target material, gelatin or hairless rat skin. The impact speed of the jet tip ranged 11 m/s
\(<V_j<\)328 m/s for the gel experiment and 52 m/s
\(<V_j<\)435 m/s for the skin experiment. Thus, the typical value of the jet power
\(P_j\) (Schramm-Baxter and Mitragotri
2004; Moradiafrapoli and Marston
2017) was
\(P_j\sim 1/8\rho \pi D_j^2V_j^3\sim\)0.04 W, where the typical values of the jet diameter
\(D_j=10\, \upmu \hbox {m}\) and the jet speed
\(V_j=100\hbox { m/s}\). The gel had reasonable transparency for visualizing the penetration dynamics of the jet, which is challenging for the non-transparent skin. However, the color contrast between the magenta ink and the rat skin was acceptable for image analysis. Thus, a cross-sectional image of the skin could visualize the overall shape of the penetration. A high-speed camera (FASTCAM SA-X, SA-Z, Photron Co., frame rate of
\(\le\)100,000 frames per second, shutter speed of
\(\sim\)0.3 µs, spatial resolution of
\(\sim\)14 µm/pixel) recorded the evolution and impact of the focused liquid jet. A backlight (KL-1600 LED, OLYMPUS Co.) was used for the high-speed imaging, and a pulse generator (Model 575 Pulse/Delay Generator, BNC) triggered both the camera and pulsed laser. Hereafter,
\(t=0\) indicates the timing of the start of laser irradiation. The standoff distance
\(H_a\) and the position of the laser focus
\(H_l\) were generally varied over the ranges 3 mm
\(\le H_a\le\)10 mm and 1 mm
\(\le H_l\le\)4 mm, respectively, except for data presented in Fig.
2c, d. For the gel experiment, the penetration depth
L(
t) and its width
W(
t) were measured by high-speed imaging. For the skin experiment, the penetration depth
\(L_s\) and its width
\(W_s\) were measured several minutes after the penetration using a digital camera (OLYMPUS TOUGH TG-620, OLYMPUS Co., with typical spatial resolution of 30 µm/pixel).
For the skin experiment, the skin from the back of a hairless rat (8 weeks old, SANKYO Lab Services) was used. The whole skin was cut into small pieces with a scalpel, and the surface of the cut skin was carefully wiped with ethanol. Each skin piece was pinned to a cork board to position it above the nozzle exit. The thickness of the skin measured by the vernier caliper was \(1.32\pm 0.12\) mm.