Droplet deformation during secondary breakup: role of liquid properties

Abstract

Deformation until the beginning of disintegration (i.e., the first rupture time) is the first stage of a droplet’s secondary breakup in a gas stream. Experiments were performed to examine the effects of density, viscosity, and surface tension of the liquid on the droplet deformation characteristics. The characteristics include droplet relative velocity, displacement, and deformation. The test liquids included molten metals along with water-like liquids, offering wide ranges of density (788–6900 kg/m$^3$), viscosity (0.86–3.63 mPa s), and surface tension (0.022–0.71 N/m). Results are presented in terms of non-dimensional numbers corresponding to each liquid property: density ratio, viscosity ratio, and surface tension ratio. It was observed that the density and surface tension ratios affected the deformation aspects, but the effect of the viscosity ratio was not significant. For the liquids excluding the metals, the relative velocity of the droplet’s leeward side was significantly smaller compared to that of the windward side, at the first rupture time. The relative velocity of the droplet was observed to be higher for the liquid with higher density ratio. At low Weber numbers ($We < {\sim }20$), lower deformation was observed for the metals compared to other liquids. For $We < {\sim }40$, higher first rupture times of low-surface-tension-ratio liquids contribute significantly to their higher displacements. Thus, it is concluded that, in addition to the Weber number, the density and surface tension ratios may need to be separately considered to characterize the droplet’s deformation phase.

Graphic Abstract

Appendix A Anomalous behaviour of SLS 0.18 wt%

Table 5 summarizes the anomalous behaviour of SLS 0.18 wt%. Experiments were performed on SLS 0.18 wt%, distilled water, and ethanol at nearly equal Weber numbers from the commonly observed ranges of Weber numbers demarking different breakup modes, as shown in Table 5. At these Weber numbers, we observed the same breakup mode for the droplets of distilled water and ethanol, whereas the droplets of SLS 0.18 wt% disintegrated in a different breakup mode. Hence, the commonly observed ranges of Weber numbers corresponding to different breakup modes of the Newtonian liquids may not be suitable for the surfactant solution considered, i.e., SLS 0.18 wt%.

Table 5 Anomalous behaviour of SLS 0.18 wt%

Fig. 16

Sequences ${\textcircled {1}}$ and ${\textcircled {2}}$ show, respectively the jellyfish and bag breakup modes for SLS 0.18 wt%. Air flow direction is from left to right