The Effect of Surface Roughness on the Contact Line and Splashing Dynamics of Impacting Droplets

Abstract Whether a droplet splashes upon impact onto a solid is known to depend not only on the fluid properties and its speed, but also on the substrate characteristics. Past research has shown that splashing is heavily influenced by the substrate roughness. Indeed, in this manuscript, we demonstrate that splashing is ruled by the surface roughness, the splashing ratio, and the dynamic contact angle. Experiments consist of water and ethanol droplets impacting onto solid substrates with varying degrees of roughness. High speed imaging is used to extract the dynamic contact angle as a function of the spreading speed for these impacting droplets. During the spreading phase, the dynamic contact angle achieves an asymptotic maximum value, which depends on the substrate roughness and the liquid properties. We found that this maximum dynamic contact angle, together with the liquid properties, the ratio of the peak to peak roughness and the surface feature mean width, determines the splashing to no-splashing threshold. In addition, these parameters consistently differentiate the splashing behaviour of impacts onto smooth hydrophilic, hydrophobic and superhydrophobic surfaces.

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Dynamics of Droplet Motion Induced by Electrowetting

Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamentals in Heat Transfer; Nanoscale Thermal Transport; Heat Transfer in Equipment; Heat Transfer in Fire and Combustion; Transport Processes in Fuel Cells and Heat Pipes; Boiling and Condensation in Macro, Micro and Nanosystems ◽

10.1115/ht2016-7331 ◽

2016 ◽

Author(s):

Yi Lu ◽

Aritra Sur ◽

Dong Liu ◽

Carmen Pascente ◽

Paul Ruchhoeft

Keyword(s):

Contact Angle ◽

High Speed ◽

Numerical Models ◽

Dynamic Contact Angle ◽

Dynamic Contact ◽

High Speed Photography ◽

Droplet Dynamics ◽

Droplet Shape ◽

Moving Contact Line ◽

Moving Contact

Electrowetting has drawn significant interests due to the potential applications in electronic displays, lab-on-a-chip devices and electro-optical switches, etc. Current understanding of electrowetting-induced droplet dynamics is hindered by the inadequacy of available numerical and theoretical models in properly handling the dynamic contact angle at the moving contact line. A combined numerical and experimental approach was employed in this work to study the spatiotemporal responses of a droplet subject to EW with both direct current and alternating current actuating signals. The time evolution of the droplet shape was measured using high-speed photography. Computational fluid dynamics models were developed by using the Volume of Fluid-Continuous Surface Force method in conjunction with a selected dynamic contact angle model. It was found that the numerical models were able to accurately predict the key parameters of the electrowetting-induced droplet dynamics.

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Dynamic Contact Angle of Modified Biopolymer Droplet on Urea Surface: Temperature Effects

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.594-595.566 ◽

2013 ◽

Vol 594-595 ◽

pp. 566-570 ◽

Cited By ~ 1

Author(s):

Yon Norasyikin Samsudin ◽

Ku Zilati Ku Shaari ◽

Zakaria Man ◽

Suriati Sufian

Keyword(s):

Contact Angle ◽

High Speed ◽

Dynamic Contact Angle ◽

Ccd Camera ◽

Dynamic Contact ◽

Droplet Impact ◽

Coating Material ◽

Coating Process ◽

Fluidised Bed ◽

Impact Behaviour

The droplet impact behaviour provides the particle coating characterization during the coating process of controlled release fertiliser. To have a good coating uniformity around the urea granules, it is necessary to enhance the wettabitily properties between the coating material and urea surface. The biopolymer material is preferred as the coating material because this polymer may degrade and will not cause any environmental impact to the environment. Various compositions of starch/urea /borate/lignin were prepared and evaluated for the wettability properties. The wettability characteristic measured is the dynamic contact angle. The high speed Charged-Couple Device (CCD) camera was used to capture the images of this droplet impact behaviour. Temperature plays an important factor during wetting stage because the coating material must be completely dried in continuous coating process in fluidised bed. From this analysis, it indicates that a composition of starch/urea/ borate (50/15/2.5) with 10% lignin has the best wettability characteristic and thus suitable to be used as a coating material. The ideal temperature for the coating process is 60°C.

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Hydrodynamic Study of an Oscillating Meniscus in a Square Mini-Channel

ASME 2009 Second International Conference on Micro/Nanoscale Heat and Mass Transfer, Volume 2 ◽

10.1115/mnhmt2009-18322 ◽

2009 ◽

Author(s):

Yajuvendra Singh Shekhawat ◽

Sameer Khandekar ◽

Pradipta Kumar Panigrahi

Keyword(s):

Contact Angle ◽

High Speed ◽

Capillary Tube ◽

Dynamic Contact Angle ◽

Dynamic Contact ◽

Glass Capillary ◽

Two Fluids ◽

Silicon Oil ◽

Static Contact ◽

Micro Systems

Miniaturized fluidic systems like MEMS may involve single-phase or multi-phase flows with oscillations/ pulsations. Understanding the hydrodynamics of such flows can help in manipulating the performance parameters and improving the efficiency of micro-systems. This work focuses on hydrodynamics of a sinusoidally oscillating meniscus in a square mini-channel. The interfacial contact line behavior of a single oscillating meniscus formed between liquid slug and air, inside the square capillary tube, has been explored. An eccentric cam follower system has been fabricated to provide sinusoidal oscillations of fluid in the square glass capillary having hydraulic diameter of 2.0 mm. Experiments are conducted with two fluids, water and silicon oil. Dynamic contact angle measurements are carried out for water at two oscillating frequencies, 0.25 Hz and 0.50 Hz using high speed videography. It is seen that an increase in the oscillating frequency increases the difference in the advancing angle and receding angle of the meniscus, with the static contact angle of water on glass surface around 21°. For silicon oil the experiments have been performed at eight different frequencies in the range of 0.20 Hz and 1.00 Hz. It is seen that the meniscus is pinned at the extreme end of the stroke, unlike that in the case of water, and there is a film of silicon oil during oscillations. The thickness of the film formed increases with increase in oscillating frequency. There is considerable difference in the hydrodynamics of silicon oil and water.

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Single Droplet Impingment: Effect of Nanoparticles

Volume 2: Fora ◽

10.1115/fedsm2008-55192 ◽

2008 ◽

Cited By ~ 2

Author(s):

Jian Shen ◽

James Liburdy ◽

Deborah Pence ◽

Vinod Narayanan

Keyword(s):

Contact Angle ◽

High Speed ◽

Early Stage ◽

Dynamic Contact Angle ◽

Dynamic Contact ◽

Droplet Surface ◽

Droplet Generator ◽

Single Droplet ◽

Spreading Dynamics ◽

Spreading Rates

This study focuses on the dynamics of water and nanofluids droplet surface impingement. Droplets are generated by a piezoelectric driven droplet generator. A high-speed-high-resolution camera is used to record droplets impacting on a smooth heated silicon surface. Droplet impact velocity, spreading diameter, spreading height and dynamic contact angle are measured by image processing. Results of water and nanofluids are compared to determine effects of nanoparticles on spreading dynamics. It is concluded that the nanofluids result in larger spreading rates, larger spreading diameters and an increase in early stage dynamic contact angle.

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The Effects of Asymmetric Micro Ratchets on Dynamic Contact Angle and Pool Boiling Performance

Volume 7: Fluids and Heat Transfer, Parts A, B, C, and D ◽

10.1115/imece2012-87176 ◽

2012 ◽

Cited By ~ 1

Author(s):

Lance Austin Brumfield ◽

Sunggook Park

Keyword(s):

Contact Angle ◽

High Speed ◽

Pool Boiling ◽

Dynamic Contact Angle ◽

Dynamic Contact ◽

Contact Angles ◽

Equilibrium Contact Angle ◽

Droplet Impingement ◽

Receding Contact ◽

Receding Contact Angle

The dynamic advancing and receding contact angles of 5μl water droplets were experimentally measured via the droplet impingement technique on a polished brass surface, one brass symmetric micro ratchet, and five brass asymmetric micro ratchet samples of varying dimensions. Droplets were released from varying heights (Weber number) and the impacts studied via high speed camera. Equilibrium advancing and receding contact angles were measured by placing a water droplet on the surfaces and tilting it. Contact angle values were then compared to an existing pool boiling model which incorporates the dynamic receding contact angle, surface roughness ratio, and equilibrium contact angle.

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Visualization of Dynamic Contact Angle

Volume 2: Heat Transfer Enhancement for Practical Applications; Heat and Mass Transfer in Fire and Combustion; Heat Transfer in Multiphase Systems; Heat and Mass Transfer in Biotechnology ◽

10.1115/ht2013-17512 ◽

2013 ◽

Author(s):

Brandon S. Field

Keyword(s):

Contact Angle ◽

Contact Line ◽

High Speed ◽

Solid Phase ◽

Capillary Rise ◽

Dynamic Contact Angle ◽

Fluid Phase ◽

Dynamic Contact ◽

Force Balance ◽

Receding Contact

Capillary rise of air-water-solid systems have been recorded with high-speed video. Glass and metal have been used as the solid phase, and the dynamic shape of the meniscus and contact angle have been characterized. The advancing and receding contact angle is of interest in computational simulations of boiling flow, and the present visualizations attempt to quantify the dynamic aspects of contact line motion. The centroid of the capillary meniscus has been tracked in order to determine the force at the contact line based on a force balance of the elevated fluid phase. The solid phase is raised and lowered in the fluid at different rates to observe advancing and receding contact lines.

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We -T classification of diesel fuel droplet impact regimes

Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rspa.2017.0759 ◽

2018 ◽

Vol 474 (2215) ◽

pp. 20170759 ◽

Cited By ~ 6

Author(s):

Hesamaldin Jadidbonab ◽

Ilias Malgarinos ◽

Ioannis Karathanassis ◽

Nicholas Mitroglou ◽

Manolis Gavaises

Keyword(s):

Surface Temperature ◽

High Speed ◽

Ambient Pressure ◽

Dynamic Contact Angle ◽

Dynamic Contact ◽

High Speed Imaging ◽

Two Phase ◽

Post Impact ◽

T Classification ◽

The Impact

A combined experimental and computational investigation of micrometric diesel droplets impacting on a heated aluminium substrate is presented. Dual view high-speed imaging has been employed to visualize the evolution of the impact process at various conditions. The parameters investigated include wall-surface temperature ranging from 140 to 400°C, impact Weber and Reynolds numbers of 19–490 and 141–827, respectively, and ambient pressure of 1 and 2 bar. Six possible post-impact regimes were identified, termed as Stick, Splash, Partial-Rebound, Rebound, Breakup-Rebound and Breakup-Stick , and plotted on the We-T map. Additionally, the temporal variation of the apparent dynamic contact angle and spreading factor have been determined as a function of the impact Weber number and surface temperature. Numerical simulations have also been performed using a two-phase flow model with interface capturing, phase-change and variable physical properties. Increased surface temperature resulted to increased maximum spreading diameter and induced quicker and stronger recoiling behaviour, mostly attributed to the change of liquid viscosity.

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Effect of Dynamic Contact Angle on Single/Successive Droplet Impingement

Volume 8: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A and B ◽

10.1115/imece2007-41603 ◽

2007 ◽

Author(s):

S. Sangplung ◽

J. A. Liburdy

Keyword(s):

Surface Tension ◽

Contact Angle ◽

Solid Surface ◽

Contact Line ◽

Dynamic Contact Angle ◽

Dynamic Contact ◽

Droplet Impingement ◽

Spreading Velocity ◽

Fluid Properties ◽

The Impact

Successive droplet impingement onto a solid surface is numerically investigated using a CFD multiphase flow model (VOF method). The main focus of this study is to better understand the hydrodynamics of the non-splash impingement process, particularly the effect of a dynamic contact angle and fluid properties along with the interaction between successive droplets while they are impinging onto a solid surface. The pre-impact droplet conditions are prescribed based on a spherical droplet diameter, velocity, and inter-droplet spacing. The molecular kinetic theory is used to model the dynamic contact angle as a function of a contact line velocity. The numerical scheme is validated against experiment results. In the impact spreading and receding processes, results are analyzed to determine the nondimensional deformation characteristics of both single and successive droplet impingements with the variation of fluid properties such as surface tension and dynamic viscosity. These characteristics include spreading ratio, spreading velocity, and a dynamic contact angle. The inclusion of a dynamic contact angle is shown to have a major effect on droplet spreading. In successive droplet impingement, the second drop causes a surge of spreading velocity and contact angle with an associate complex recirculating flow near the contact line after it initially impacts the preceding droplet when it is in an advancing condition. This interaction is less dramatic when the first drop is receding or stationary. The surface tension has the most effect on the maximum spreading radius in both single and successive droplet impingements. In contrast to this, the viscosity directly affects the damping of the spreading-receding process.

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