Displacement of fluid droplets from solid 

surfaces in low-Reynolds-number shear flows

Dimitrakopoulos, P.; Higdon, J. J. L.

doi:10.1017/s0022112096004788

Cited by 96 publications

(99 citation statements)

References 32 publications

(60 reference statements)

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“…The velocity of a single particle was evaluated from a PIV (particle image velocity) analysis of two consecutive frames. Unlike the usual bidirectional symmetric circulation observed inside a falling water droplet in air or the Marangoni flow inside a sessile water droplet evaporating on a surface 20,27 , only one direction of circulation is visible in Figure 4b because of the asymmetric shear flow around the sessile droplet 28 . High-speed recordings were made through the observation window using a high-speed camera system (Photron Fastcam PCI1024) with a maximum image resolution of 1,024×1,024 pixels, mounted on a binocular microscope (Olympus SZX10).…”

Section: Methodsmentioning

confidence: 79%

Mechanism of supercooled droplet freezing on surfaces

Jung

Tiwari

Doan³

et al. 2012

Nat Commun

592

391

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understanding ice formation from supercooled water on surfaces is a problem of fundamental importance and general utility. superhydrophobic surfaces promise to have remarkable 'icephobicity' and low ice adhesion. Here we show that their icephobicity can be rendered ineffective by simple changes in environmental conditions. Through experiments, nucleation theory and heat transfer physics, we establish that humidity and/or the flow of a surrounding gas can fundamentally switch the ice crystallization mechanism, drastically affecting surface icephobicity. Evaporative cooling of the supercooled liquid can engender ice crystallization by homogeneous nucleation at the droplet-free surface as opposed to the expected heterogeneous nucleation at the substrate. The related interplay between droplet roll-off and rapid crystallization is also studied. overall, we bring a novel perspective to icing and icephobicity, unveiling the strong influence of environmental conditions in addition to the accepted effects of the surface conditions and hydrophobicity.

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Section: Methodsmentioning

confidence: 79%

Mechanism of supercooled droplet freezing on surfaces

Jung

Tiwari

Doan³

et al. 2012

Nat Commun

592

391

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“…The issue of the shear rate at which the liquid detaches from the channel is related to studies of moving droplets with contact angle hysteresis [22,23].…”

mentioning

confidence: 99%

Shear Flow Pumping in Open Micro- and Nanofluidic Systems

2007

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We propose to drive open microfluidic systems by shear in a covering fluid layer, e.g., oil covering water-filled chemical channels. The advantages as compared to other means of pumping are simpler forcing and prevention of evaporation of volatile components. We calculate the expected throughput for straight channels and show that devices can be built with off-the-shelf technology. Molecular dynamics simulations suggest that this concept is scalable down to the nanoscale.Progressive miniaturization and integration of chemical and physical processes into microfluidic systems, so-called "chemical chips", is expected to permit cheap mass production and to facilitate the handling of much smaller quantities than standard laboratory equipment [1,2,3]. Most currently built microfluidic systems contain the liquids in closed pipes and use body forces (e.g., gravity or centrifugal forces) [4], electroosmosis, capillary wicking, or simply applied pressure to generate flow. Driving these systems is relatively straightforward, but there are major drawbacks. Small pipes get easily clogged (in particular while processing biological fluids which contain large molecules such as proteins or DNA), driving gets increasingly difficult with reduced cross section, and fabrication more challenging.Open microfluidic systems are a second line of development which can overcome these problems. As demonstrated experimentally [5,6] and theoretically [7,8], fluids can be guided on flat solid surfaces by wettability patterns, e.g., hydrophilic stripes on an otherwise hydrophobic substrate. Several methods to generate flow in such "chemical channels" have been discussed: capillary forces (i.e., wicking into channels or motion due to wettability gradients) [5], electrowetting [9,10], thermally generated Marangoni forces (e.g., in arrays of local heaters) [11,12], body forces, surface acoustic waves [13], or combinations of these. While capillary forces can only induce flow over relatively short distances, electrode and heater arrays require complex sample structures. Surface acoustic waves can only drive drops which are large compared with the wavelength (in general on the micron scale), and high rotation velocities are required to obtain sufficiently strong centrifugal forces.As illustrated in Fig. 1 we propose to use shear, generated in a covering fluid layer (e.g., oil on water), to induce flow over long length scales, e.g., across the whole device. A top plate moving with respect to the bottom patterned substrate generates shear in the oil which fills the gap between the two plates and, in turn, the oil will induce flow in a chemical channel covered with, e.g., water. This principle can be realized in a device with two circular plates rotating relative to each other and the chemical patterning can be implemented with standard printing techniques [14,15]. Design and scalability of shear driven systems is similar to body force driven devices and the same type of surface tension driven pearling instabilities occur in straight channels [8]. In b...

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“…Though no modeling work applicable to water droplet removal at the cathode GDL/GFC interfaces as in PEMFCs has been reported in the literature at the time when this work was carried out, the subject of displacing liquid droplets from solid surfaces is a fundamental problem of fluid mechanics and has attracted considerable interests due to their important applications in chemical process technologies such as coating-flow manufacturing processes and enhanced oil recovery (see, e.g., Dussan V. and Chow 1983, Dussan V. 1985, 1987Dimitrakopoulos and Higdon 1997, 1998, 2001. The first two studies by Dussan V. and co-workers, which consider only small drops with small contact angles and employ analytical techniques (made possible by using lubrication theory and asymptotic analysis) for solving the resultant governing equations, focused on the influence of gravity in dislodging the drops from non-horizontal surfaces.…”

Section: Predicting the Onset Of Water Droplet Instability At The Gasmentioning

confidence: 99%

“…The third study by Dussan V. evaluated the ability of the creeping motion of the surrounding fluid to remove the drops by sweeping them across the solid surface. More recently, Dimitrakopoulos and Higdon (1997, 1998, 2001) employed the spectral finite element method to numerically investigate the displacement or stability of fluid droplets from solid surfaces in shear and viscous pressure driven flows with vanishing Reynolds number (that is, creeping flows in which Stokes equations apply). They determined the optimal droplet shape and contact line position that allows the highest flow rate for which the droplet can adhere to the surface.…”

Section: Predicting the Onset Of Water Droplet Instability At The Gasmentioning

confidence: 99%

Final report on LDRD project : elucidating performance of proton-exchange-membrane fuel cells via computational modeling with experimental discovery and validation.

Wang

Pasaogullari

Noble³

et al. 2006

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In this report, we document the accomplishments in our Laboratory Directed Research and Development project in which we employed a technical approach of combining experiments with computational modeling and analyses to elucidate the performance of hydrogen-fed proton exchange membrane fuel cells (PEMFCs). In the first part of this report, we document our focused efforts on understanding water transport in and removal from a hydrogen-fed PEMFC. Using a transparent cell, we directly visualized the evolution and growth of liquid-water droplets at the gas diffusion layer (GDL)/gas flow channel (GFC) interface. We further carried out a detailed experimental study to observe, via direct visualization, the formation, growth, and instability of water droplets at the GDL/GFC interface using a speciallydesigned apparatus, which simulates the cathode operation of a PEMFC. We developed a simplified model, based on our experimental observation and data, for predicting the onset of water-droplet instability at the GDL/GFC interface. Using a state-of-the-art neutron imaging instrument available at NIST (National Institute of Standard and Technology), we probed liquid-water distribution inside an operating PEMFC under a variety of operating conditions and investigated effects of evaporation due to local heating by waste heat on water removal. Moreover, we developed computational models for analyzing the effects of micro-porous layer on net water transport across the membrane and GDL * Current address: Connecticut Global Fuel Cell Center, and Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269-5233.4 anisotropy on the temperature and water distributions in the cathode of a PEMFC. We further developed a two-phase model based on the multiphase mixture formulation for predicting the liquid saturation, pressure drop, and flow maldistribution across the PEMFC cathode channels.In the second part of this report, we document our efforts on modeling the electrochemical performance of PEMFCs. We developed a constitutive model for predicting proton conductivity in polymer electrolyte membranes and compared model prediction with experimental data obtained in our laboratory and from literature. Moreover, we developed a one-dimensional analytical model for predicting electrochemical performance of an idealized PEMFC with small surface over-potentials. Furthermore, we developed a multi-dimensional computer model, which is based on the finite-element method and a fully-coupled implicit solution scheme via Newton's technique, for simulating the performance of PEMFCs. We demonstrated utility of our finite-element model by comparing the computed current density distribution and overall polarization with those measured using a segmented cell. In the last part of this report, we document an exploratory experimental study on MEA (membrane electrode assembly) degradation. AcknowledgmentsThis work was funded by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. We would...

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Displacement of fluid droplets from solid surfaces in low-Reynolds-number shear flows

Cited by 96 publications

References 32 publications

Mechanism of supercooled droplet freezing on surfaces

Mechanism of supercooled droplet freezing on surfaces

Shear Flow Pumping in Open Micro- and Nanofluidic Systems

Final report on LDRD project : elucidating performance of proton-exchange-membrane fuel cells via computational modeling with experimental discovery and validation.

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