Name: Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films (Video) | JoVE Journal
Uploaded: 2018-08-18T13:00:00.000+00:00
Duration: 7 min 8 s
Description: Explore droplet impact dynamics on flowing liquid films using high-speed imaging and controlled wave generation. This method enables detailed analysis of short timescale phenomena in fluid mechanics.

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5 minutes

This work was sponsored by the Petroleum Technology Development Fund (PTDF, Nigeria) and the Engineering and Physical Sciences Research Council, UK, through the Programme Grant MEMPHIS (grant number EP/K003976/1). The authors also appreciate fruitful discussions with Dr. Zhizhao Che.

1. Experimental Rig Setup
NOTE: See Figure 1.
<ol>
	<li>Falling film unit
	<ol>
		<li>Start by cleaning the substrate (glass) surface with a clean, soft cloth. Ensure no dirt is adhered to its surface, which would alter the liquid properties.</li>
		<li>Set the pivot of the glass substrate to the desired inclination angle. An inclination angle, &#946;, of 15&#730; was used in this work.</li>
		<li>Switch on the electric pump and ensure a normal liquid flow on the film surface to further clean the glass substrate. For this work, the test liquid was deionized water.</li>
		<li>Ensure the entire surface of the substrate is wetted.</li>
		<li>Measure the film flow rate using the flow meter. For this work, the flow rate was varied between 1.667 x 10-3 and 10 x 10-3 m3/s with the corresponding film Reynolds number, Re = &#961;q/w&#181;, ranging between 55.5 and 333. w is the falling film width, 0.30 m.</li>
		<li>Gradually adjust the valves on the flow connection to obtain the desired flow rate on the glass substrate.</li>
		<li>Adjust the set of micrometer step at the film inlet to the corresponding Nusselt film thickness value for the chosen flow rate, to avoid a hydraulic jump at the film inlet or a backflow of air into the distribution chamber.</li>
		<li>Manually siphon all air in the distribution chamber to obtain a uniform flow downstream on the film surface.</li>
	</ol>
	</li>
	<li>Film control unit
	<ol>
		<li>Ensure that the function generator is connected to the solenoid valve through a non-latching relay via a data acquisition card (DAC).</li>
		<li>Switch on both the solenoid valve and the function generator.</li>
		<li>Set the function generator to the desired forcing frequency. In this work, frequencies of 2 and 3 Hz were used.</li>
		<li>Choose the desired wave signal (sine wave, sawtooth wave, square wave, etc.). In this work, a sine wave signal was used. Figure 2A and 2B show the contrast between an uncontrolled film and a controlled film.</li>
	</ol>
	</li>
	<li>Droplet generation system
	<ol>
		<li>Attach a clean plastic tubing to a water-filled syringe.</li>
		<li>Insert the syringe into the droplet generator.</li>
		<li>Affix a syringe-needle of a chosen size (depending on the desired droplet diameter) to the other end of the plastic tubing. The droplet diameter range studied was between 0.0023 to 0.0044 m.</li>
		<li>Adjust the fall height of the drop above the film surface. In this work, the drop&#39;s fall height was varied from 0.005 to 0.45 m, giving impact speed between 0.30 &#177; 0.02 - 2.96 &#177; 0.06 m/s.</li>
		<li>Similarly, set the streamwise impact point of the drop from the film inlet. This was set to 0.3 m in this work to ensure the waves are well-formed prior to impact.</li>
		<li>Set a desired flow rate for the syringe pump.</li>
		<li>Adjust the flow rate to achieve a droplet generation frequency greater than the wavelength of the waves formed on the film surface; to ensure drops successively impinge on different regions of the controlled film. See Figure 2C; with an enlargement of a singular waveform in Figure 2D to show the dissimilarities in the flow profile beneath each region19,20.</li>
	</ol>
	</li>
	<li>High-speed imaging setup
	<ol>
		<li>Place the camera on a tripod stand (or any other suitable arrangement).</li>
		<li>Select the macro lens with desired focal length and connect this to the camera.</li>
		<li>Switch on the high-speed camera and ensure direct focus on the film surface. Align the camera at 7&#730; and 12&#730; horizontal and vertical deviations respectively to the film surface. This gives an excellent side-view image of the impact process, resulting in a resolution of 67.5 &#181;m/pixel and 46.6 &#181;m/pixel in the streamwise and spanwise directions, respectively.</li>
		<li>Adjust the focus of the camera-lens (at the largest aperture) using a calibration item&#160;placed exactly on the droplet impact spot.</li>
		<li>Once a sharp focus has been obtained, reduce the aperture to ensure only a small amount of light enters the camera.</li>
		<li>Set the desired frame rate, resolution, and shutter speed of the high-speed camera. A frame rate of 5000 fps, 800 x 600 resolution, aperture size 1/16, and a shutter speed of 1 &#181;s were used in this work.</li>
		<li>Place the light diffuser in front of the light source, as shown in Figure 1C, to ensure the light is uniformly diffused across the imaging region.</li>
		<li>Power on the light source to confirm the uniform spreading of light over the imaging area.</li>
	</ol>
	</li>
</ol>
2. Calibration
NOTE: See Figure 3.
<ol>
	<li>Put a ruler in the film flow direction (exactly on the spot of impact) and obtain snapshots of measured points on the film surface.</li>
	<li>Repeat 2.1 but with the ruler in the spanwise direction.</li>
	<li>Use the above to obtain the spatial resolutions on the film surface.</li>
</ol>
3. Video Recording and Data Acquisition
<ol>
	<li>Once film flow is established on the rig, start the syringe pump and observe the impact of the dripping drops on the film surface.</li>
	<li>Start the function generator and observe the production of spatiotemporally regular waves on the film surface.</li>
	<li>Ensure successive drops are impacting the different regions of the controlled film surface.</li>
	<li>Observe the post-triggering frame number and set this to approximately half of the video length to adequately capture the impact.</li>
	<li>Power on the light source and trigger the image capturing once an impact occurs.</li>
	<li>Power off the light source once image capturing is complete to avoid overheating of the liquid film.</li>
	<li>Visually analyze the obtained snapshot on the computer screen. Check to see if the impact has occurred on one of the flat film, capillary wave, or wave hump regions.</li>
	<li>Trim down the video to the portion showing the impact process and save the frame range in a video/image format.</li>
	<li>Repeat 3.5 - 3.8 and record individual impact on all regions on the film surface, viz. solitary hump, capillary waves, and flat film.</li>
</ol>
4. Image Post-processing and Analysis
<ol>
	<li>Place a ruler in the field of view and calculate the spatial resolution by counting how many pixels fit across 1 cm. Using the calibration image, obtain a scale factor for image dimension measurement.</li>
	<li>Compare the outcomes of the impact process on the different impact regions from the high-speed images. Check to see notable differences.</li>
	<li>Using a suitable MATLAB image-processing routine, measure the characteristics features of the product of the impact process: viz. in the splashing mode, measure the crown height, diameter, wall thickness, tilt angle, crown-facing direction, number and size distribution of ejected secondary droplets.</li>
	<li>Carry out similar quantitative analyses as 4.3 above for the low-Weber impacts. Count the pinch-off time of satellite drops from the time-framed images and measure the apex length and width&#160;of the column formed in partial coalescence before pinch-off of secondary drops. Measure the size of ejected secondary drops. Count the number of cascade in a repeated pinch-off process.</li>
	<li>Observe all qualitative differences in each region.</li>
</ol>

<ol><li>Yarin, A. L. <a target="_blank" href="https://www.annualreviews.org/doi/abs/10.1146/annurev.fluid.38.050304.092144">Drop impact dynamics: Splashing, spreading, receding, bouncing….</a> Annual Review of Fluid Mechanics. 38, 159-192 (2006).</li>
<li>Rein, M. <a target="_blank" href="http://iopscience.iop.org/article/10.1016/0169-5983(93)90106-K/meta">Phenomena of liquid drop impact on solid and liquid surfaces.</a> Fluid Dynamics Research. 12 (2), 61-93 (1993).</li>
<li>Liang, G., Mudawar, I. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/S0017931016304136">Review of mass and momentum interactions during drop impact on a liquid film.</a> International Journal of Heat and Mass Transfer. 101, 577-599 (2016).</li>
<li>Dam, D. B., Le Clerc, C. <a target="_blank" href="http://aip.scitation.org/doi/10.1063/1.1773551">Experimental study of the impact of an ink-jet printed droplet on a solid substrate.</a> Physics of Fluids. 16 (9), 3403-3414 (2004).</li>
<li>Worthington, A. M. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aWorthington%2C+AM+%22A+study+of+splashes%22">A study of splashes</a>. , Longmans, Green, and Company. (1908).</li>
<li>Edgerton, H. E., Killian, J. R. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aEdgerton%2C+HE+%22Flash%21+Seeing+the+unseen+by+ultra-high-speed+photography%22">Flash! Seeing the unseen by ultra-high-speed photography</a>. , CT Branford Co. (1954).</li>
<li>Josserand, C., Thoroddsen, S. T. <a target="_blank" href="https://www.annualreviews.org/doi/abs/10.1146/annurev-fluid-122414-034401">Drop impact on a solid surface.</a> Annual Review of Fluid Mechanics. 48, 365-391 (2016).</li>
<li>Kolinski, J. M., Mahadevan, L., Rubinstein, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Lift-off+instability+during+the+impact+of+a+drop+on+a+solid+surface%5BTitle%5D)+AND+%22Physical+Review+Letters%22%5BJournal%5D)">Lift-off instability during the impact of a drop on a solid surface.</a> Physical Review Letters. 112 (13), 134501(2014).</li>
<li>Hobbs, P. V., Osheroff, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Splashing+of+drops+on+shallow+liquids%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Splashing of drops on shallow liquids.</a> Science. 158 (3805), 1184-1186 (1967).</li>
<li>Adomeit, P., Renz, U. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/S0301932299000798">Hydrodynamics of three-dimensional waves in laminar falling films.</a> International Journal of Multiphase Flow. 26 (7), 1183-1208 (2000).</li>
<li>Blanchette, F., Bigioni, T. P. <a target="_blank" href="https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/dynamics-of-drop-coalescence-at-fluid-interfaces/B66DFE98A0DC1B74FB892E614A503B39">Dynamics of drop coalescence at fluid interfaces.</a> Journal of Fluid Mechanics. 620, 333-352 (2009).</li>
<li>Wang, A. B., Chen, C. C. <a target="_blank" href="https://aip.scitation.org/doi/abs/10.1063/1.1287511">Splashing impact of a single drop onto very thin liquid films.</a> Physics of Fluids. 12 (9), 2155-2158 (2000).</li>
<li>Che, Z., Deygas, A., Matar, O. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Impact+of+droplets+on+inclined+flowing+liquid+films%5BTitle%5D)+AND+%22Physical+Review+E%22%5BJournal%5D)">Impact of droplets on inclined flowing liquid films.</a> Physical Review E. 92 (2), 023032(2015).</li>
<li>Craster, R. V., Matar, O. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dynamics+and+stability+of+thin+liquid+films%5BTitle%5D)+AND+%22Reviews+of+Modern+Physics%22%5BJournal%5D)">Dynamics and stability of thin liquid films.</a> Reviews of Modern Physics. 81 (3), 1131(2009).</li>
<li>Adebayo, I. T., Matar, O. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Droplet+impact+on+flowing+liquid+films+with+inlet+forcing%3A+the+splashing+regime%5BTitle%5D)+AND+%22Soft+Matter%22%5BJournal%5D)">Droplet impact on flowing liquid films with inlet forcing: the splashing regime.</a> Soft Matter. 13 (41), 7473-7485 (2017).</li>
<li>Chang, H. H., Demekhin, E. A. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aChang%2C+HH+%22Complex+wave+dynamics+on+thin+films%22">Complex wave dynamics on thin films</a>. 14, Elsevier. (2002).</li>
<li>Liu, J., Gollub, J. P. <a target="_blank" href="https://aip.scitation.org/doi/abs/10.1063/1.868232">Solitary wave dynamics of film flows.</a> Physics of Fluids. 6 (5), 1702-1712 (1994).</li>
<li>Benjamin, T. B. <a target="_blank" href="https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112057000373">Wave formation in laminar flow down an inclined plane.</a> Journal of Fluid Mechanics. 2 (6), 554-573 (1957).</li>
<li>Kalliadasis, S., Ruyer-Quil, C., Scheid, B., Velarde, M. G. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aKalliadasis%2C+S+%22Falling+liquid+films%22">Falling liquid films</a>. 176, Springer Science & Business Media. (2011).</li>
<li>Adebayo, I., Xie, Z., Che, Z., Matar, O. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Doubly+excited+pulse+waves+on+thin+liquid+films+flowing+down+an+inclined+plane%3A+An+experimental+and+numerical+study%5BTitle%5D)+AND+%22Physical+Review+E%22%5BJournal%5D)">Doubly excited pulse waves on thin liquid films flowing down an inclined plane: An experimental and numerical study.</a> Physical Review E. 96 (1), 013118(2017).</li>
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</ol>

The authors have nothing to declare.

In this section, we provide a few tips necessary to ensure qualitative results are obtained from the protocol. First, the glass substrate on which the liquid film flows must be kept completely dirt-free to ensure the properties of the liquid film are kept uncompromised. This is achievable by regular cleaning (probably using a suitable detergent, and wiped off over a tray to avoid dissolution into the system). Similarly, there should be a regular replacement of the whole test-liquid after some experimental rounds, to guarantee accurate results.
Secondly, the fluid-distribution chamber must be well-meshed and also kept air-tight to ensure the outflowing liquid film is uniform. This can be done by manually siphoning air out of the distribution box before each experiment. The use of micrometer-steps at the film inlet is also advised to set the gap-height at the film inlet to the exact film thickness predicted by the Nusselt estimate of the film flow at the corresponding Reynolds number. This will prevent a hydraulic jump or backflow at the inlet.
The operation of the solenoid valve must also always be checked and ascertained properly. This is because an appropriate pulsation of the flow is required to ensure the production of the forced waves. This could be checked from the regular clicking sound of the solenoid valve as well as a perceived pulsation along the connection pipes. The liquid flow rate into the syringe pump must also be set carefully to ensure the droplets are ejected in a dripping manner, avoiding any pre-acceleration before falling.
Appropriate calibration of the high-speed camera must be ensured to obtain very accurate results. The aperture size must also be carefully chosen considering parameters like the depth of field, exposure time and overall image brightness. For the camera triggering during video recording, users are also required to estimate how many frames should be recorded before triggering. This may vary with individuals, depending on the drop impact time, hence, several trial tests for practicing are recommended before actual measurements. Similarly, the light source must be properly arranged and well-diffused to minimize shadows in the image.
It is important to note and remember that the main focus of the study is the contributions of waves to the impact dynamics of the falling drops, hence the formation of regular wave structures is essential to an accurate study of the underlying physics. In scenarios where the wave structures are observed to quickly transition to three-dimensional structures, it is advised that the substrate inclination angle be reduced14,19 to facilitate a slower transition of the wave structures.
One limitation of the technique is observed in the absence of a measuring device specifying the actual instantaneous film thickness on each region of impact. This would have provided additional details on the overall observed phenomena.
In summary, the procedure outlined in this report can also be used to study simple wave evolution dynamics, while the high-speed imaging system described can be applied to many research fields with fast dynamics such as liquid drop break-up21,22/coalescence23, granular jets24, etc. where important phenomena are observed at a micro timescale.

Droplet impact is a very common phenomenon in nature and attracts attention from any curious observer1. It constitutes an active research area due to its numerous applications including spray-cooling, fire-suppression, inkjet-printing, spray-coating, deposition of solder bumps on printed circuit boards, the design of internal combustion engines, surface-cleaning, and cell-printing2. Its application extends also to agriculture, e.g., sprinkling irrigation and crop spraying3,4. Pioneering work dates back to the 19th century, with the work of Worthington5, while major advances have only been recently made due to the emergence of high-speed imaging6. Since then, several studies have been carried out; using different types of impact surfaces ranging from solids7,8, shallow,9 and deep liquid pools10,11 to thin films12,13.
However, despite the large volume of research on droplet impact on liquid surfaces (i.e., shallow and deep pools and quiescent films), impact on flowing thin liquid films has not received as much attention. In addition, hitherto, studies have neglected the contributions of spatial structures of waves to droplet impact dynamics.
In this report, we present a detailed experimental procedure to investigate the droplet impact process on flowing films whose dynamics are influenced by inlet-forcing of the liquid flow rate; below, we refer to them as &#39;controlled&#39; films. We find that these have numerous applications in multiphase industries (e.g., in cooling towers, in distillation columns, and also in the annular flow regime observed in two-phase flows), especially as film control has become an important step in the intensification of both heat and mass transfer in many process industries14. The interested reader is referred to our previous work15 for more details on the results of our research efforts on this.
This application of frequency oscillations of the inlet flow rate results in the formation of regular waves on the film surface. We focus on the solitary wave family, which is essentially characterized by widely-separated narrow peaks and is preceded by a series of front-running capillary waves16,17,18. We study the outcome of the impacts associated with the three main parts of the solitary wave structure: the &#39;flat film&#39;, &#39;wave hump&#39;, and front-running &#39;capillary wave&#39; regions. We also contrast these results with those associated with uncontrolled flowing films. Our results show that the stochastic nature of wave appearance on the uncontrolled film markedly affects the outcome of drop impact, with the separate regions of the controlled film also showing new mechanisms, which we have detailed both qualitatively and quantitatively.
In the previous paper15, using the same procedure, we studied the effect of film control on droplet impact dynamics in the splashing regime. The obtained results showed both quantitative and qualitative differences in the crown morphology (height, diameter, wall thickness, tilt angle, and direction) as well as the number and size distribution of the ejected secondary droplets.
In this report, we describe the designed set-up to understand the critical role played by these spatial structures in the droplet impact dynamics and also present succinct details of our findings not only in the splashing regime but also for other outcomes of droplet impact (viz. bouncing, sliding, partial/total coalescence). By following the standard protocol described below, the effect of film control on the droplet impact dynamics can be studied in a reproducible fashion.

<table><tbody><tr><td>Function generator</td><td>GW INSTEK</td><td>AFG 2005 Series, Digital. Geo0852266</td><td>Produces a varied type of wave signals, ranging from sine, square to saw-tooth wave at different frequencies (0.1 Hz - 5 MHz).</td></tr><tr><td>Syringe pump</td><td>Braintree Scientific Inc.</td><td>Bs-8000 /225540</td><td></td></tr><tr><td>Solenoid valve</td><td>SMC-VXD</td><td>2142A.&lt;br/&gt; 0AE-5001</td><td>Series-pilot-operated-two-port</td></tr><tr><td>Relay</td><td>Takamisara</td><td>A5W-K.&lt;br/&gt; 154424C-03L</td><td></td></tr><tr><td>Electric pump</td><td>Clarke SP</td><td>SPE1200SS 1</td><td></td></tr><tr><td>Flow meter</td><td>RS Component</td><td>CYNERGY3 UF25B 14011600040110</td><td>Measurement range: 0.2-25 L/min</td></tr><tr><td>Micrometer step</td><td>RS Component</td><td>Micrometer Head</td><td>0.01 mm/0 -13 mm</td></tr><tr><td>High-speed camera</td><td>Olympus</td><td>I-SPEED 3.</td><td>Capable of recording at up to 100, 000 frames per second.</td></tr><tr><td>Light source</td><td>TLC Electrical supplies</td><td>IP54 -black</td><td>Double enclosed halogen floodlight. Rating 500 W.</td></tr><tr><td>Light diffusor</td><td>OptiGraphix</td><td>DFPMET</td><td>250 &amp;mu;m thickness</td></tr><tr><td>Glass substrate</td><td>Instrument Glasses Ltd</td><td>Soda Lime Float Glass; 570 mm x 300 mm x 4 mm</td><td>Flatness tolerance 0.02/0.04.</td></tr><tr><td>Macro-lenses</td><td>(a) Nikon&lt;br/&gt; (b) Sigma</td><td>(a) AF-Micro-Nikkor 60 mm f/2.8 D&lt;br/&gt; (b) 105 mm f/2.8 Macro-Ex</td><td></td></tr><tr><td>Test-liquid</td><td>De-ionized water from the Imperial College Analytical Lab.</td><td>Standard solution&lt;br/&gt; (AnalaR)</td><td></td></tr></tbody></table>

film control to study contributions of waves to droplet impact dynamics on thin flowing liquid films

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

A protocol to study the contributions of waves to droplet impact dynamics on flowing liquid films is presented.

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous ...

film-control-to-study-contributions-waves-to-droplet-impact-dynamics

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JoVE Journal

Engineering

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Explore droplet impact dynamics on flowing liquid films using high-speed imaging and controlled wave generation. This method enables detailed analysis of short timescale phenomena in fluid mechanics.

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