Name: film control to study contributions of waves to droplet impact dynamics on thin flowing liquid films
Uploaded: 2018-08-18T13:00:00.000+00:00
Duration: 7 min 8 s
Description: Watch this Scientific Journal Video about Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films at JoVE.com

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>

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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

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.

Watch this Scientific Journal Video about Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films at JoVE.com

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

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 ...

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Nanyang Technological University

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7.4K Views.  Imperial College London. A protocol to study the contributions of waves to droplet impact dynamics on flowing liquid films is presented.

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

Fast Imaging Technique to Study Drop Impact Dynamics of Non-Newtonian Fluids

In the field of fluid mechanics, many dynamical processes not only occur over a very short time interval but also require high spatial resolution for detailed observation, scenarios that make it challenging to observe with conventional imaging systems. One of these is the drop impact of liquids, which usually happens within one tenth of millisecond. To tackle this challenge, a fast imaging technique is introduced that combines a high-speed camera (capable of up to one million frames per second) with a macro lens with long working distance to bring the spatial resolution of the image down to 10 &#181;m/pixel. The imaging technique enables precise measurement of relevant fluid dynamic quantities, such as the flow field, the spreading distance and the splashing speed, from analysis of the recorded video. To demonstrate the capabilities of this visualization system, the impact dynamics when droplets of non-Newtonian fluids impinge on a flat hard surface are characterized. Two situations are considered: for oxidized liquid metal droplets we focus on the spreading behavior, and for densely packed suspensions we determine the onset of splashing. More generally, the combination of high temporal and spatial imaging resolution introduced here offers advantages for studying fast dynamics across a wide range of microscale phenomena.

Drop impact of non-Newtonian fluids is a complex process since different physical parameters influence the dynamics over a very short time (less than one tenth of a millisecond). A fast imaging technique is introduced in order to characterize the impact behaviors of different non-Newtonian fluids.

In the field of fluid mechanics, many dynamical processes not only occur over a very short time interval but also require high spatial resolution for ...

Taking Advantage of Reduced Droplet-surface Interaction to Optimize Transport of Bioanalytes in Digital Microfluidics

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; platforms. Often, droplet motion relies on the wetting of a surface, directly associated with the application of an electric field; surface interactions, however, make motion dependent on droplet contents, limiting the breadth of applications of the technique.
Some alternatives have been presented to minimize this dependence. However, they rely on the addition of extra chemical species to the droplet or its surroundings, which could potentially interact with droplet moieties. Addressing this challenge, our group recently developed Field-DW devices to allow the transport of cells and proteins in DMF, without extra additives.
Here, the protocol for device fabrication and operation is provided, including the electronic interface for motion control. We also continue the studies with the devices, showing that multicellular, relatively large, model organisms can also be transported, arguably unaffected by the electric fields required for device operation.

The protocol for fabrication and operation of field dewetting devices (Field-DW) is described, as well as the preliminary studies of the effects of electric fields on droplet contents.

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; ...

Visualization of High Speed Liquid Jet Impaction on a Moving Surface

Two apparatuses for examining liquid jet impingement on a high-speed moving surface are described: an air cannon device (for examining surface speeds between 0 and 25 m/sec) and a spinning disk device (for examining surface speeds between 15 and 100 m/sec). The air cannon linear traverse is a pneumatic energy-powered system that is designed to accelerate a metal rail surface mounted on top of a wooden projectile. A pressurized cylinder fitted with a solenoid valve rapidly releases pressurized air into the barrel, forcing the projectile down the cannon barrel. The projectile travels beneath a spray nozzle, which impinges a liquid jet onto its metal upper surface, and the projectile then hits a stopping mechanism. A camera records the jet impingement, and a pressure transducer records the spray nozzle backpressure. The spinning disk set-up consists of a steel disk that reaches speeds of 500 to 3,000 rpm via a variable frequency drive (VFD) motor. A spray system similar to that of the air cannon generates a liquid jet that impinges onto the spinning disc, and cameras placed at several optical access points record the jet impingement. Video recordings of jet impingement processes are recorded and examined to determine whether the outcome of impingement is splash, splatter, or deposition. The apparatuses are the first that involve the high speed impingement of low-Reynolds-number liquid jets on high speed moving surfaces. In addition to its rail industry applications, the described technique may be used for technical and industrial purposes such as steelmaking and may be relevant to high-speed 3D printing.

Two experimental devices for examining liquid jet impingement on a high-speed moving surface are described: an air cannon device and a spinning disk device. The apparatuses are used to determine optimal approaches to the application of liquid friction modifier (LFM) onto rail tracks for top-of-rail friction control.

Two apparatuses for examining liquid jet impingement on a high-speed moving surface are described: an air cannon device (for examining surface speeds ...

Capillary-based Centrifugal Microfluidic Device for Size-controllable Formation of Monodisperse Microdroplets

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal microfluidic device. W/O microdroplets have recently been used in powerful methods that enable miniaturized chemical experiments. Therefore, developing a versatile method to yield monodisperse W/O microdroplets is needed. We have developed a method for generating monodisperse W/O microdroplets based on a capillary-based centrifugal axisymmetric co-flowing microfluidic device. We succeeded in controlling the size of microdroplets by adjusting the capillary orifice. Our method requires equipment that is easier-to-use than with other microfluidic techniques, requires only a small volume (0.1-1 &#181;l) of sample solution for encapsulation, and enables the production of hundreds of thousands number of W/O microdroplets per second. We expect this method will assist biological studies that require precious biological samples by conserving the volume of the samples for rapid quantitative analysis biochemical and biological studies.

Here, we demonstrate a simple production method for size-controllable, monodisperse, water-in-oil (W/O) microdroplets using a capillary-based centrifugal microfluidic device. This method requires only a small sample volume and enables high-yield production. We expect this method will be useful for rapid biochemical and cellular analyses.

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal ...

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion co-flow device to generate uniform high-viscosity droplets in a low-viscosity fluid. The microfluidic capillary device has a common co-flow structure with its exit connecting to a wider tube. Elongated droplets of the low-viscosity fluid are first encapsulated by the high-viscosity fluid in the co-flow structure. As the elongated low-viscosity droplets flow through the exit, which is treated to be wetted by the low-viscosity fluid, phase inversion is then induced by the adhesion of the low viscosity droplets to the tip of the exit, which results in the subsequent inverse encapsulation of the high-viscosity fluid. The size of the resultant high-viscosity droplets can be adjusted by changing the flow rate ratio of the low-viscosity fluid to the high-viscosity fluid. We demonstrate several typical examples of the generation of high-viscosity droplets with a viscosity up to 11.9 Pas, such as glycerol, honey, starch, and polymer solution. The method provides a simple and straightforward approach to generate monodisperse high-viscosity droplets, which may be used in a variety of droplet-based applications, such as materials synthesis, drug delivery, cell assay, bioengineering, and food engineering.

A phase-inversion co-flow device is demonstrated to generate monodisperse high-viscosity droplets above 1 Pas, which is difficult to realize in droplet microfluidics.

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion ...

Chemistry

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. This paper proposes a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation. The polydimethylsiloxane (PDMS) chip was fabricated using the multi-layer soft lithography method. Hexadecane containing surfactant was used as the continuous phase, and PEGDA with the ultraviolet (UV) photo-initiator was the dispersed phase. Surfactants allowed the local surface tension to drop and formed a more cusped tip which promoted breaking into tiny micro-droplets. As the pressure of dispersed phase was constant, the size of droplets became smaller with increasing continuous phase pressure before dispersed phase flow was broken off. As a result, droplets with size variation from 1 &#181;m to 80 &#181;m in diameter could be selectively achieved by changing the pressure ratio in two inlet channels, and the average coefficient of variation was estimated to be below 7%. Furthermore, droplets could turn into micro-beads by UV exposure for photo-polymerization. Conjugating biomolecules on such micro-beads surface have many potential applications in the fields of biology and chemistry.

Generation of Size-controlled Poly ethylene Glycol Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. ...

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.

Two different methods for characterizing the incipient particle motion of a single bead as a function of the sediment bed geometry from laminar to turbulent flow are presented.

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to ...

Impacts of Free-falling Spheres on a Deep Liquid Pool with Altered Fluid and Impactor Surface Conditions

Vertical impacts of spheres on clean water have been the subject of numerous water entry investigations characterizing cavity formation, splash crown ascension and Worthington jet stability. Here, we establish experimental protocols for examining splash dynamics when smooth free-falling spheres of varying wettability, mass, and diameter impact the free surface of a deep liquid pool modified by thin penetrable fabrics and liquid surfactants. Water entry investigations provide accessible, easily assembled and executed experiments for studying complex fluid mechanics. We present herein a tunable protocol for characterizing splash height, flow separation metrics, and impactor kinematics, and representative results which might be acquired if reproducing our approach. The methods are applicable when characteristic splash dimensions remain below approximately 0.5 m. However, this protocol may be adapted for greater impactor release heights and impact velocities, which augurs well for translating results to naval and industry applications.

This protocol demonstrates the basic experimental configuration for water entry experiments with free-falling spheres. Methods for the alteration of liquid surface with penetrable fabrics, the preparation of chemically non-wetting spheres, and steps for splash visualization and data extraction are discussed.

Vertical impacts of spheres on clean water have been the subject of numerous water entry investigations characterizing cavity formation, splash crown ...

Measuring the Interaction Force Between a Droplet and a Super-hydrophobic Substrate by the Optical Lever Method

The goal of this paper is to investigate the interaction force between droplets and super-hydrophobic substrates in the air. A measurement system based on an optical lever method is designed. A millimetric cantilever is used as a force sensitive component in the measurement system. Firstly, the force sensitivity of the optical lever is calibrated using electrostatic force, which is the critical step in measuring interaction force. Secondly, three super-hydrophobic substrates with different grid fractions are prepared with nanoparticles and copper grids. Finally, the interaction forces between droplets and super-hydrophobic substrates with different grid fractions are measured by the system. This method can be used to measure the force on the scale of sub-micronewton with a resolution on the scale of nanonewton. The in-depth study of the contact process of droplets and super-hydrophobic structures can help to improve the production efficiency in coating, film and printing. The force measurement system designed in this paper can also be used in other fields of microforce measurement.

The protocol aims to investigate the interaction between droplets and super-hydrophobic substrates in the air. This includes calibrating the measurement system and measuring the interaction force at super-hydrophobic substrates with different grid fractions.

The goal of this paper is to investigate the interaction force between droplets and super-hydrophobic substrates in the air. A measurement system based on ...

High Throughput Analysis of Liquid Droplet Impacts

Experimental studies of liquid drop impacts on surfaces are often restricted in their scope due to the large range of possible experimental parameters such as material properties, impact conditions, and experimental configurations. Compounding this, drop impacts are often studied using data-rich high-speed photography, so that it is difficult to analyze many experiments in a detailed and timely manner. The purpose of this method is to enable efficient study of droplet impacts with high-speed photography by using a systematic approach. Equipment is aligned and calibrated to produce videos that can be accurately processed by a custom image processing code. Moreover, the file structure setup and workflow described here ensure efficiency and clear organization of data processing, which is carried out while the researcher is still in the lab. The image processing method extracts the digitized outline of the impacting droplet in each frame of the video, and processed data are stored for further analysis as required. The protocol assumes that a droplet is released vertically under gravity, and impact is recorded by a camera viewing from side-on with the drop illuminated using shadowgraphy. Many similar experiments involving image analysis of high-speed events could be addressed with minor adjustment to the protocol and equipment used.

This protocol enables efficient collection of experimental high-speed images of liquid drop impacts, and speedy analysis of those data in batches. To streamline these processes, the method describes how to calibrate and set up apparatus, generate an appropriate data structure, and deploy an image analysis script.

Experimental studies of liquid drop impacts on surfaces are often restricted in their scope due to the large range of possible experimental parameters ...