Abstract
Binary droplet collisions exhibit a wide range of outcomes, including coalescence and stretching separation, with a transition between these two outcomes arising for high Weber numbers and impact parameters. Our experimental study elucidates the effect of viscosity on this transition, which we show exhibits inertial (viscosity-independent) behaviour over an order-of-magnitude-wide range of Ohnesorge numbers. That is, the transition is not always shifted towards higher impact parameters by increasing droplet viscosity, as it might be thought from the existing literature. Moreover, we provide compelling experimental evidence that stretching separation only arises if the length of the coalesced droplet exceeds a critical multiple of the original droplet diameters (3.35). Using this as a criterion, we provide a simple but robust model (without any arbitrarily chosen free parameters) to predict the coalescence/stretching-separation transition.
Highlights
Binary droplet collisions exhibit a wide range of outcomes, including bouncing, permanent coalescence and eventual breakup (Ashgriz & Poo 1990; Qian & Law 1997; Pan et al 2016, 2019)
Note that the first column in figure 4 is an example of RS, which occurs for much lower impact parameters than stretching separation (SS), as seen in figure 1(b)
We have comprehensively demonstrated the effect of Ohnesorge number on the C–SS transition in binary droplet collisions
Summary
Binary droplet collisions exhibit a wide range of outcomes, including bouncing, permanent coalescence and eventual breakup (Ashgriz & Poo 1990; Qian & Law 1997; Pan et al 2016, 2019). We are primarily interested in the transition from coalescence (C) to stretching separation (SS), which occurs when two equal-size droplets collide at sufficient We and B, as illustrated by the red dashed line in figure 1(b). Ashgriz & Poo (1990) proposed a new criterion, which states that separation occurs only if the effective kinetic energy exceeds the surface energy in the regions of interaction. We observe that SS occurs only if the stretched droplets reach a particular non-dimensional critical length, regardless of We, Oh and B We use this experimentally observed critical length to propose a simple but robust model for the C–SS transition, in which all numerical parameters are directly specified by readily obtainable experimental data (i.e. the model does not contain any arbitrarily chosen free parameters). The offset B was varied by applying a frequency shift between the two streams of droplets, as explained by Gotaas et al (2007a)
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