Noncoalescence in the Oblique Collision of Fluid Jets

When two jets of fluid collide, they can “bounce” off each other, due to a thin film of air which keeps them separated. We describe the phenomenon of stable noncoalescence between two jets of the same fluid, colliding obliquely with each other. Using a simple experimental setup, we carry out a parametric study of the bouncing jets by varying the jet diameter, velocity, angle of inclination, and fluid viscosity, which suggests that the contact time of bouncing jets scales as the square root of the normal Weber number We. A dimensionless parameter $K=(\mathrm{We}\sqrt{\mathrm{Re}}/\sin \alpha {)}^{1/2}$, where Re is the normal Reynolds number and $\alpha$ the angle of inclination of the jets, quantitatively captures the transition of colliding jets from bouncing to coalescence. This parameter draws parallels between jet coalescence and droplet splashing and indicates that the transition is governed by a surface instability. Stable and continuous noncoalescence between fluid jets makes it a good platform for experimental studies of the interaction between fluid interfaces and the properties of the interfacial air films.

Figure 1

(a) Two silicone oil jets (kinematic viscosity 17.9 cSt, diameter 0.97 mm, velocity $0.53\mathrm{m}/\mathrm{s}$, angle 45° with the vertical) collide obliquely and bounce off each other and (b) the jets coalesce when the velocity is increased to $0.64\mathrm{m}/\mathrm{s}$.

Figure 2

(a) log-log plot between dimensionless contact time against normal Weber number (We) for $\nu =4.37$ cSt (green), $\nu =8.52$ cSt (black), $\nu =17.9$ cSt (red), $\nu =46.6$ cSt (blue) and 75% by weight glycerol solution (red diamonds). Solid line is the best fit on the data with $\mathrm{\text{slope}}=0.49$ ($\mathrm{rmse}=0.12$) and $R^2=0.82$. Dashed line is a guide to the eye with a slope of 0.5. Inset: A typical image from the experiment. $L$ is the length of the contact region, $\alpha$ the angle the jets make with the vertical, and $\delta$ is the change in diameter of the jet during the collision. (b) $T/D^{3/2}\sin \alpha$ for all silicone oil data indicating velocity independence. Color code is the same as that in (a).

Figure 3

(a) Transition velocity $V_{\mathrm{cr}}$ versus $\alpha$ for three different diameters ($\nu =17.9$ cSt). Solid lines represent experimental data and dashed lines represent transition velocity calculated from $K_{cr}=6.1$. (Inset) Median bouncing life-time versus jet velocity $V$ for $\nu =17.9$ cSt, $D=1.35\mathrm{mm}$, $\alpha =38.8°$. Red arrow indicates $V_{\mathrm{cr}}$. Error bars are of length equal to one mean absolute deviation. (b) $K=(\mathrm{We}\sqrt{\mathrm{Re}}/\sin \alpha {)}^{1/2}$ versus jet angle $\alpha$. Open symbols represent bouncing and solid symbols represent coalescence. Solid horizontal line represents the $K_{\mathrm{cr}}=6.1$, while the shaded region is of thickness equal to twice the standard deviation ($V_{\mathrm{cr}}=0.4$, $n=32$). Color and size code for the symbols is same as that in Fig. 2.

Figure 4

Transition from bouncing to coalescence. Sequence of high speed images shows coalescence triggering in the top portion of the contact region (top panel) and coalescence triggering in the bottom portion of the contact region (bottom panel). Videos were taken at 20 000 frames per second. White arrows point to the location where coalescence occurs. $\nu =17.9$ cSt, $V=0.59\mathrm{m}/\mathrm{s}$, $D=0.81\mathrm{mm}$, $\alpha =62.7°$.