“…), the non-interaction regions (see figure 1 a ) are large enough that the stretching energy (mainly the kinetic energy of the non-interaction regions) leads to SS, as seen in the last column of figure 4 (Ashgriz & Poo 1990; Pan et al. 2019). Figure 4.Dynamics of 2 % HPMC () droplet collisions at for different impact parameters , from head-on collision exhibiting RS (first column), through C (second and third columns), to the onset of SS (fourth column).…”
Section: Resultsmentioning
confidence: 96%
“…1992; Pan et al. 2019; Suo & Jia 2020). However, the viscous forces have always been treated by adjustable parameters subject to arbitrary choices, while the inertial and the regions of resistant capillary forces continued to be subjectively specified without consideration of the difference in the intermediate geometries that might arise from different combinations of , and , as seen in the second and third columns of figure 2.…”
Section: Introductionmentioning
confidence: 99%
“…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). Understanding the underlying fluid physics of collision outcomes, and the transitions between them, is both of significant fundamental interest and tremendous practical importance.…”
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.
“…), the non-interaction regions (see figure 1 a ) are large enough that the stretching energy (mainly the kinetic energy of the non-interaction regions) leads to SS, as seen in the last column of figure 4 (Ashgriz & Poo 1990; Pan et al. 2019). Figure 4.Dynamics of 2 % HPMC () droplet collisions at for different impact parameters , from head-on collision exhibiting RS (first column), through C (second and third columns), to the onset of SS (fourth column).…”
Section: Resultsmentioning
confidence: 96%
“…1992; Pan et al. 2019; Suo & Jia 2020). However, the viscous forces have always been treated by adjustable parameters subject to arbitrary choices, while the inertial and the regions of resistant capillary forces continued to be subjectively specified without consideration of the difference in the intermediate geometries that might arise from different combinations of , and , as seen in the second and third columns of figure 2.…”
Section: Introductionmentioning
confidence: 99%
“…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). Understanding the underlying fluid physics of collision outcomes, and the transitions between them, is both of significant fundamental interest and tremendous practical importance.…”
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.
“…Droplet collisions have been extensively studied in past decades (Ashgriz & Poo 1990;Jiang, Umemura & Law 1992;Qian & Law 1997;Estrade et al 1999;Brenn, Valkovska, & Danov 2001;Pan, Law & Zhou 2008;Pan, Chou & Tseng 2009;Zhang & Law 2011;Tang, Zhang, & Law 2012;Kwakkel, Breugem, & Boersma 2013;Huang & Pan 2015;Li 2016;Pan et al 2016Pan et al , 2019Sommerfeld & Kuschel 2016;Hu et al 2017;Al-Dirawi & Bayly 2019;Huang, Pan & Josserand 2019;Chubynsky et al 2020) due to significant relevance to a variety of systems in natural and technological situations. Examples are seen in raindrop formation (Gunn 1965;Strangeways 2006), medical therapy (May 1973;Feng et al 2016), disease transmission (Tellier 2009;Gralton et al 2011) and combustion processes in engines (Chiu 2000;Zhang et al 2016).…”
Section: Introductionmentioning
confidence: 99%
“…Typically, the collision outcomes of two identical droplets can be mapped in the regime diagrams depicted by a Weber number (We = ρ l U 2 D/σ ) and an impact parameter (B = χ /D = sin θ). They show coalescence after minor deformation (I), bouncing (II), coalescence after substantial deformation (III), reflexive separation (IV), stretching separation (V) (Ashgriz & Poo 1990;Qian & Law 1997) and rotational separation (VI) (Pan et al 2019). Here We indicates the ratio of impact inertia and surface tension force and B represents the effect of the colliding angle (θ) between the droplets.…”
In droplet impacts, transitions between coalescence and bouncing are determined by complex interplays of multiple mechanisms dominating at various length scales. Here we investigate the mechanisms and governing parameters comprehensively by experiments and scaling analyses, providing a unified framework for understanding and predicting the outcomes when using different fluids. Specifically, while bouncing had not been observed in head-on collisions of water drops under atmospheric conditions, it was found in our experiments to appear on increasing the droplet diameter sufficiently. Contrarily, while bouncing was always observed in head-on impacts of alkane drops, we found it to disappear on decreasing the diameter sufficiently. The variations are related to gas draining dynamics in the inter-droplet film and suggest an easier means for controlling bouncing as compared to alternating the ambient pressure usually sought. The scaling analysis further shows that for a given Weber number, enlarging droplet diameter or fluid viscosities, or lowering surface tension contributes to a larger characteristic minimum thickness of the gas film, thus enhancing bouncing. The key dimensionless group
$(O{h_{g,l}},\;O{h_l},\;{A^\ast })$
is identified, referred to as the two-phase Ohnesorge number, the Ohnesorge number of liquid and the Hamaker constant, respectively. Our thickness-based model indicates that as
${h^{\prime}_{m,c}} > 21.1{h_{cr}}$
, where
${h^{\prime}_{m,c}}$
is the maximum value of the characteristic minimum film thickness
$({h_{m,c}})$
and
${h_{cr}}$
is the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when
$1.2O{h_{g,l}}/(1 - 2O{h_l}) > \sqrt[3]{{{A^\ast }}}$
, a fully developed bouncing regime occurs, thereby yielding a lower coalescence efficiency. The transitional Weber number is found universally to be 4.
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