TY - JOUR
T1 - Transitions of bouncing and coalescence in binary droplet collisions
AU - Huang, Kuan Ling
AU - Pan, Kuo Long
N1 - Publisher Copyright:
© 2021 The Author(s). Published by Cambridge University Press.
PY - 2021/12/10
Y1 - 2021/12/10
N2 - 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 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 21.1 hcr, where is the maximum value of the characteristic minimum film thickness and is the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when [3]A, a fully developed bouncing regime occurs, thereby yielding a lower coalescence efficiency. The transitional Weber number is found universally to be 4.
AB - 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 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 21.1 hcr, where is the maximum value of the characteristic minimum film thickness and is the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when [3]A, a fully developed bouncing regime occurs, thereby yielding a lower coalescence efficiency. The transitional Weber number is found universally to be 4.
KW - breakup/coalescence
KW - drops
UR - http://www.scopus.com/inward/record.url?scp=85117228367&partnerID=8YFLogxK
U2 - 10.1017/jfm.2021.797
DO - 10.1017/jfm.2021.797
M3 - 期刊論文
AN - SCOPUS:85117228367
SN - 0022-1120
VL - 928
JO - Journal of Fluid Mechanics
JF - Journal of Fluid Mechanics
M1 - A7
ER -