The Dynamics of Marangoni-Driven Local Film Drainage between Two Drops

Yeo, Leslie; Matar, Omar; Ortiz, Emma; Hewitt, Geoffrey F.

doi:10.1006/jcis.2001.7743

Cited by 49 publications

(28 citation statements)

References 27 publications

(38 reference statements)

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“…This is because the self-similarity has been studied well in the case of drop pinch-off (18-21) but rarely studied in the opposite process of coalescence (22). As a matter of fact, as demonstrated in Fig.…”

Section: Resultsmentioning

confidence: 99%

Dimensional crossover in the coalescence dynamics of viscous drops confined in between two plates

Yokota

Okumura

2011

Proc. Natl. Acad. Sci. U.S.A.

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Coalescence of liquid drops is a daily phenomenon familiar to everybody and is related to many fields from biology to astronomy and also related to a variety of practical problems in industry. However, the detailed physical understanding of the dynamics has been revealed only recently with the aid of high-speed camera, high-performance computer, and scaling analysis. In this study, coalescence of a viscous drop to a bath of the same liquid is studied in a confined space. This is because dealing with a small amount of liquid drops becomes increasingly important (e.g., in industrial and biological applications). Here, the aqueous drop and bath are surrounded by low-viscosity oil and sandwiched by two parallel plates of the cell. We quantify experimentally the width of a neck that bridges the drop and the bath during coalescence. As a result, we find that the neck width increases linearly with time at short times, but the dynamics slows down significantly at longer times. Thanks to simple and original scaling arguments, we clearly show that this transition of the dynamics with time in a single coalescence event is brought about by a crossover from a three-dimensional viscous dynamics for a spherical drop to a quasi two-dimensional one for a disk drop. In addition, we report an unusual type of coalescence that is possibly caused by naturally accumulated electric charge in the confined geometry and whose dynamics seems self-similar. F rom daily experiences, everybody knows a liquid drop falling onto a bath of the same liquid merges to the bath. A spectacular aspect of this mundane phenomenon of coalescence of droplets was already studied as early as 1885 by Thomson and Newall (1). It is important in various problems, such as fusion of cells in biology and of galaxies in the universe, and in a large number of industrial applications, such as emulsion stability, inkjet printing, and others (2). Accordingly, it is still an active area of research (3-5).In particular, the recent advances in technology, associated with high-speed camera, image analysis, and simulation, together with theoretical development, are leading us to a new phase of understanding. Recently, it has been established that the coalescence dynamics driven by capillary force is balanced by viscous force at shorter times (or in viscous drops) and by inertial force at longer times (or in less-viscous drops) (6, 7). More recently, this has been confirmed also in two-dimensional coalescence (8) and the possibility of a new inertial regime is reported (9). Furthermore, still another new viscous regime, which can be rather regarded as a film bursting, is reported in ref. 10.Here, we study coalescence of a droplet of radius R in a confined geometry of a Hele-Shaw cell (in between two plates whose distance D is smaller than the droplet size R), which will be relevant in many practical situations where a small amount of liquid has to be manipulated (e.g., microfluidics and biological applications). We followed the dynamics of the half of the neck width r (see Fi...

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Section: Resultsmentioning

confidence: 99%

Dimensional crossover in the coalescence dynamics of viscous drops confined in between two plates

Yokota

Okumura

2011

Proc. Natl. Acad. Sci. U.S.A.

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“…[68]. Therefore, during the drainage of the gas layer, the magnitude of the lubrication force F lubrication can be estimated at different thicknesses of the gas layer, as shown in Figure 12.…”

Section: Iii7 Lubrication Gas Layer and Force Analysismentioning

confidence: 99%

“…Estimation from the flow in the gas layer, the liquid film, and the droplet Due to the small thickness of the gas layer and the liquid film relative to the drainage length of the gas layer in the tangential direction, the flow in the gas layer and in the liquid film can be approximated using lubrication theory. The approximation is also applied in a penetration depth h d in the droplet, and the velocity gradient vanishes and the velocity approaches the droplet tangential speed at the penetration depth [67,68], as shown in Figure 11. We also neglect deformations of the film-gas interface in the present, approximate analysis:…”

Section: Iii7 Lubrication Gas Layer and Force Analysismentioning

confidence: 99%

Impact of droplets on inclined flowing liquid films

2015

Self Cite

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The impact of droplets on an inclined falling liquid film is studied experimentally using highspeed imaging. The falling film is created on a flat substrate with controllable thicknesses and flow rates. Droplets with different sizes and speeds are used to study the impact process under various Ohnesorge and Weber numbers, and film Reynolds numbers. A number of phenomena associated with droplet impact are identified and analysed, such as bouncing, partial coalescence, total coalescence, and splashing. The effects of droplet size, speed, as well the film flow rate are studied culminating in the generation of an impact regime map. The analysis of the lubrication force acted on the droplet via the gas layer shows that a higher flow rate in the liquid film produces a larger lubrication force, slows down the drainage process, and increases the probability of droplet bouncing. Our results demonstrate that the flowing film has a profound effect on the droplet impact process and associated phenomena, which are markedly more complex than those accompanying impact on initially quiescent films.

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“…(5) are solved numerically using the Method of Lines [11,12] subject to the following initial conditions:…”

Section: Dynamic Microjet Simulationmentioning

confidence: 99%

High frequency AC electrosprays: mechanisms and applications

Yeo¹,

Chang²

2006

WIT Transactions on Engineering Sciences, Vol 52

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We have recently discovered a new electrospray phenomenon using high frequency AC electric fields beyond 10 kHz which behaves very distinctly from its DC electrospray counterpart. Experimental and theoretical findings have demonstrated that plasma charging plays a significant role in the electrohydrodynamically-driven interfacial dynamics of the electrospray. These findings show that Faradaic generation of interfacial plasma polarized layers can produce a net normal Maxwell stress that is responsible for the generation of aerosol drops. Experimental results of this plasma polarization phenomenon are presented and scaling theory is employed to elucidate the underlying plasma mechanism responsible for the phenomenon. The AC electrospray is capable of producing micron and sub-micron electroneutral drops or polymeric fibers and is a promising rapid and portable technology for drug encapsulation or biomaterials synthesis for controlled release respiratory drug delivery, wound care therapy or tissue/orthopaedic engineering.

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The Dynamics of Marangoni-Driven Local Film Drainage between Two Drops

Cited by 49 publications

References 27 publications

Dimensional crossover in the coalescence dynamics of viscous drops confined in between two plates

Dimensional crossover in the coalescence dynamics of viscous drops confined in between two plates

Impact of droplets on inclined flowing liquid films

High frequency AC electrosprays: mechanisms and applications

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