Spreading and contact-line arrest dynamics of impacting oxidized liquid-metal droplets

Gallium based room temperature liquid metal alloys present an exciting opportunity for research due to their attractive material properties. These alloys can readily remain liquid at room temperature and exhibit unusual phenomena at the air-liquid interface due to oxidation of the gallium. We present a phenomenological study of droplet impacts on a solid substrate measuring the spread parameter ${\xi }_{max}=D_{max}/D_0$ as function of the Weber number, and by extension, the impact velocity, as well as the spread time of the droplets. In order to characterize the surface behavior, we utilized direct measurement of surface forces using a glass probe and a tensiometer, finding the effective surface tension to be ${\sigma }_{\mathrm{eff}}=628\pm 37\mathrm{mN}/\mathrm{m}$. Finally, we developed a model of spreading that scales the spread factor ${\xi }_{max}$ with the Weber number to the power of $\frac{1}{2}$.

Figure 1

(Left) RTLM droplet in flight with 2 mm scale bar. Shape is maintained through entire falling sequence. (Right) Water droplet in flight with 1 mm scale bar.

Figure 2

(a) Impact sequence of liquid metal droplets at $\mathrm{We}=42.6$ (top) and DI water droplets at $\mathrm{We}=40.0$ (bottom) on hydrophobic substrate. Time stamps are 0, 0.4, 0.9, 1.7, 3.5, and 15.4 ms for both sequences. Scale bar indicates 1 mm. (b) The same sequence normalized by $D_{max}$; note that ${\xi }_{max}$ is not the same for both datasets.

Figure 3

Experimental setup for droplet impact experiments.

Figure 4

(a) Droplet spread time vs impact diameter $D_0$. (b) Droplet impact regions as defined by Schiaffino and Sonin [27].

Figure 5

Comparison of spread factor scaling models and impact data. Green, red, and orange dashed lines are the Pasandideh-Fard model for normal surfaces [22] for advancing contact angles of 1°, 90°, and 180° respectively; the solid blue line is the Clanet model for water on a superhydrophobic surface [24]; and the solid black line is this work. The markers indicate the impact data and are differentiated by needle size.

Figure 6

Spread factor $\xi$ vs time for six different impacts. Red represents the 20G needle with filled circles corresponding to 22.8 μl droplet at $\mathrm{We}=22.3$ and open circles are 21.8 μl at $\mathrm{We}=106.9$. Blue represents the 25G needle with filled triangles at 10.6 μl and $\mathrm{We}=27.4$ and open triangles at 10.6 μl and $\mathrm{We}=117.8$. Green represents the 30G needles with filled squares at 6.2 μl at $\mathrm{We}=27.4$ and open squares at 6.5 μl and $\mathrm{We}=114.1$.