Ultrafast X-Ray Phase-Contrast Imaging of the Initial Coalescence Phase of Two Water Droplets

We report an ultrafast x-ray phase-contrast imaging study of the early merging dynamics of two water drops in air. Owing to the edge-enhancement capability, the high penetrability, and the unprecedented temporal and spatial resolutions offered by this new x-ray technique, the coalescence singularity of two water drops was revisited. A finite initial contact radius was identified and the evolvement of the trapped toroidal air bubble was studied for the first time. Despite the existence of this finite initial contact radius, the subsequent meniscus radius followed power laws which agree with theoretical predictions for the inviscid regime.

Figure 1

Water drops coalescence process recorded with ultrafast high-resolution x-ray phase-contrast imaging with 472 ns exposure time. The images give an overview of the two drops with enhanced boundaries through the phase-contrast mechanism. The scale bar is 1 mm long. The numbers indicate the time instances relative to the start of merging (SOM).

Figure 2

Close-up views of the contact region show the liquid bridges (meniscus) in the early stage of the merging. Notice the different vertical and horizontal scale bars. The vertical and horizontal scale bars are 50 and $100\mu \mathrm{m}$ long, respectively. The highly concave air-water interfaces with trapped toroidal air bubbles are visible as indicated by the guide.

Figure 3

Phase-contrast imaging contrast comparison of drops when they are merging or simply spatially overlapping. Panel (a) and (c) shows the experimental and simulation images when the drops are spatially overlapping and panel (b) and (d) show the corresponding merging case. The image contrast between two scenarios is clearly different, which is crucial for the determination of the coalescence time. The scale bar is $50\mu \mathrm{m}$ long.

Figure 4

Meniscus radius, meniscus gap width, and the toroidal bubble height as a function of time. Symbols are experimental data and the lines are power laws fittings. The inset shows data close to the coalescence onset. The meniscus gap width and toroidal bubble height were extracted only up to 0.4 ms due to the ambiguity of their definitions when the bubble escaped. The error bars for both temporal and spatial measurement are about the sizes of the symbols in the inset.