The authors would like to acknowledge the College of Engineering and Computer Sciences (CECS) at the University of Central Florida for funding this project, Joshua Bom and Chris Souchik for splash imagery and Nicholas Smith for valuable feedback.

1. Configuring the experiment for vertical impacts
<ol>
	<li>Fill a transparent water tank of dimensions approximately 60 cm x 30 cm x 36 cm (length x weight x depth) with 32 L of water and mount a meter ruler (&#8216;visual scale&#8217;) vertically inside the container such that the base sits atop the fluid, as seen in Figure 1a. 
	NOTE: Depth and width of the tank must be greater than 20 times the diameter of the largest spheres used in the experiment to ensure wall effects are negligible9. Greater entry speeds than those described here will require greater tank depth. The visual scale used to determine drop heights and calibration of tracking software is discussed in section 7.</li>
	<li>Place an additional meter ruler under the water, which can act to magnify dimensions. This visual scale is used for calibrating tracking software for underwater measurements.</li>
	<li>Construct a hinged platform (&#8216;release mechanism&#8217;) that suspends spheres above the fluid and rotates downward, to achieve tangential acceleration greater than gravity at the impactor location when released, as seen in Figure 1a. Rapid rotation is achieved by connecting the hinged platform to the center of the supporting component using elastic bands. The result is an unsupported and non-rotating impactor. 
	NOTE: The platform is easily fabricated with 3D printer.</li>
	<li>For impact trials, place thumb to base of hinged platform and rotate it 90&#176; to a horizontal position for placement of spheres above the fluid. 
	NOTE: Retraction is triggered when thumb is released from base of the platform.</li>
	<li>Affix the release mechanism to a retort stand, such that the device can be adjusted to various heights.</li>
	<li>Place the retort stand next to the tank such that the release mechanism is within the same depth plane as the visual scale. Add a weight to the base of the retort stand as needed to prevent toppling.</li>
	<li>Adjust the release mechanism to the maximum desired experimental drop height. This is necessary for optimal splash visualization as discussed in section 6 and ensures the splash characteristics of interest are always within the viewing frame of the camera.</li>
	<li>Attach a multi-LED light to an articulating arm such that the light is mounted above the camera, looking down onto the splash zone. Ambient light alone is insufficient to illuminate the scene at the high frame rates needed to extract splash kinematics. 
	NOTE: One can never have too much light.</li>
	<li>Place a black screen at the back of the water tank to aid splash and cavity visualization as seen in Figure 2.</li>
	<li>Place a glass-protecting shock absorber, such as a closed-cell sponge, at the bottom of water tank and affix with weights to prevent resurfacing. 
	NOTE: The height of the fluid in the tank should be such that the sphere does not interact with shock absorber prior to air cavity pinch off10.</li>
</ol>
2. Controlling dimensionless parameters
<ol>
	<li>Conduct experiments with smooth spheres of various masses and diameters. For this, polyoxymethylene (e.g., Delrin) coin-making balls work particularly well and have no mold part line. Measure masses and diameters with an analytical balance and Vernier caliper respectively.</li>
	<li>Conduct experiments over a range of heights H to generate impact velocities <img alt="Equation 1" src="/files/ftp_upload/59300/59300eq1.jpg" /> where <img alt="Equation 2" src="/files/ftp_upload/59300/59300eq2.jpg" />m/s2 is the acceleration due to gravity. Measure height with the visual scale within the camera frame. 
	NOTE: Use the Auto-Tracking feature in the video analysis tool as discussed in section 7 to measure impact velocities.</li>
	<li>Conduct experiments with fluid mixtures of water and suitable surfactants (e.g., glycerin or soap) to modify surface tension. Measure surface tension with a surface tensiometer.</li>
	<li>Calculate Reynolds numbers <img alt="Equation 3" src="/files/ftp_upload/59300/59300eq3.jpg" /> and Weber numbers <img alt="Equation 4" src="/files/ftp_upload/59300/59300eq4.jpg" />, where &#961; is the density of the fluid, D is the sphere diameter, &#956; is the dynamic viscosity of the fluid and &#963; the surface tension of the fluid.</li>
</ol>
3. Maintaining sanitary experimental conditions
<ol>
	<li>Conduct experiments while wearing industrial nitrile gloves and retrieve spheres from water tank with a sanitized scoop. 
	CAUTION: Skin naturally produces oils which can affect the wettability of impactors and taint fluid conditions.</li>
	<li>Clean the spheres with 99% isopropyl alcohol and allow to dry for 1 min in between trials to preclude the influence of impurities.</li>
	<li>If using fabrics that break apart during impact, replace the water in the tank after every trial if scraps cannot be manually collected.</li>
	<li>At the end of experiment, empty the tank and leave it to dry.</li>
	<li>Before an experiment, clean the tank with water to remove any impurities.</li>
</ol>
4. Layering the surface with penetrable fabrics
<ol>
	<li>Separate the fabric into square or round plies in preparation for impact trials. Use a Vernier caliper to obtain compressed thickness of the fabric. 
	NOTE: Fabric thickness will change when wet.</li>
	<li>Gently rest the dry fabric atop the surface of the liquid pool. Ensure that the plies do not begin descent before impactor release and replace fabrics immediately after collision.</li>
	<li>Use a sanitized scoop to position the fabric below the hinged platform before releasing spheres.</li>
	<li>(Optional) Conduct the following tests using a fabric sample for material characterization.
	<ol>
		<li>Perform tensile testing using a tensile tester to determine the elastic modulus of the sample.</li>
		<li>Use a digital microscope to obtain a microscopic view of the fabric and determine fiber length using an imaging tool.</li>
	</ol>
	</li>
</ol>
5. Preparing chemically hydrophobic spheres
<ol>
	<li>Spray the hydrophobic base coat approximately 15&#8211;30 cm from the sphere surface. Avoid soaking the surface. Let it dry for 1&#8210;2 min before adding additional coatings. Apply two more base coats. Allow it to dry for 30 min before applying the top coat. 
	NOTE: The number of additional surface may vary based on recommendations from the product manufacturer.</li>
	<li>Spray the hydrophobic top coat approximately 15&#8211;30 cm from the surface. Avoid soaking the surface. Let it dry for 1&#8211;2 min before adding additional coatings. Apply two or three more coatings of top coat. Allow to dry for 30 min for light use and 12 h for full use. 
	NOTE: The number of additional surface coats may vary based on recommendations from the product manufacturer.</li>
	<li>After approximately 20 trials, the hydrophobic coating becomes compromised due to excessive handling. Remove coating with 99% isopropyl and repeat steps 5.1 and 5.2.</li>
</ol>
6. Synchronizing cameras for splash visualization
<ol>
	<li>Place a high-speed camera with a suitable lens perpendicular to the impact axis and in-line with the surface of the fluid. 
	NOTE: A 55 mm prime lens provides a good starting point.</li>
	<li>Where fabrics are to be used, add an additional high-speed camera to the experiment to provide a top-down view of the impacts, as seen in Figure 1b.</li>
	<li>Synchronize multiple cameras to a computer using the following steps.
	<ol>
		<li>Connect both output terminals of the horizontal camera to both input terminals of the additional camera using BNC cables.</li>
		<li>Connect the trigger switch to the horizontal camera only.</li>
		<li>Plug Ethernet cables from both cameras into an off-network router connected to the computer. 
		NOTE: In the absence of a router, connect Ethernet cables of cameras to separate computers.</li>
	</ol>
	</li>
	<li>In the video acquisition software, configure the cameras with the following settings. Set frame rate to a minimum of 1,000 fps, set screen resolution to the desired resolution. Set the shutter speed to 1 per frame second and set trigger mode to end.</li>
	<li>From maximum release height, conduct a series of test trials to ensure that the Worthington jets are within the video frame.</li>
	<li>Adjust the camera position and focus accordingly until the desired visualization quality is achieved.</li>
	<li>After recording, extract kinematic and geometric measurements from videos using a suitable video analysis tool. Use Tracker, an open source analysis tool or any software of comparative capability.</li>
</ol>
7. Digitizing impact kinematics with tracker software
<ol>
	<li>Select calibration stick from the Tracker toolbox and match it to the visual scale (Figure 2a), making the stick as long as possible.</li>
	<li>Click calibration stick and set the scaling value to the length of the visual scale spanned by the stick. That is, if the calibration stick spans 1 cm on visual scale, set scaling value to 1. 
	NOTE: This ensures measurements taken from software are in the order of centimeters.</li>
	<li>Toggle video playback by clicking start and stop and set video to the desired frame.</li>
	<li>Select measuring stick from the Tracker toolbox and extract splash crown height k, cavity width b, cavity depth l, and Worthington jet height h, as seen in Figure 2b,c. 
	NOTE: The measuring stick is adjustable at both ends and can be used simultaneously with other toolbox selections.</li>
	<li>Select protractor from the Tracker toolbox and measure the separation angle q of fluid with respect to the impactor, as seen in Figure 2b. The protractor is adjustable at both ends and can be used simultaneously with other toolbox selections.</li>
	<li>Select the Auto-Tracking feature in the software to record temporal position and velocity data. When tracking is interrupted due to lack of clarity in the video, use manual tracking until clarity is obtained and auto-tracking is resumed.</li>
</ol>

<ol>
	<li>Watson, D. A., Stephen, J. L., Dickerson, A. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Jet+amplification+and+cavity+formation+induced+by+penetrable+fabrics+in+hydrophilic+sphere+entry.">Jet amplification and cavity formation induced by penetrable fabrics in hydrophilic sphere entry.</a> Physics of Fluids. 30, 082109 (2018).</li><li>Truscott, T. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Cavity dynamics of water entry for spheres and ballistic projectiles. , (2009).</li><li>Truscott, T., Techet, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Water+entry+of+spinning+spheres.">Water entry of spinning spheres.</a> Journal of Fluid Mechanics. 625, 135 (2009).</li><li>Techet, A., Truscott, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Water+entry+of+spinning+hydrophobic+and+hydrophilic+spheres.">Water entry of spinning hydrophobic and hydrophilic spheres.</a> Journal of Fluids and Structures. , 716 (2011).</li><li>Zhao, S., Wei, C., Cong, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Numerical+investigation+of+water+entry+of+half+hydrophilic+and+half+hydrophobic+spheres.">Numerical investigation of water entry of half hydrophilic and half hydrophobic spheres.</a> Mathematical Problems in Engineering. 2016, 1-15 (2016).</li><li>Worthington, A. M., Cole, R. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+with+a+liquid+surface+studied+by+the+aid+of+instantaneous+photography.">Impact with a liquid surface studied by the aid of instantaneous photography.</a> Philosophical Transactions of the Royal Society of London. 137, 137 (1897).</li><li>Worthington, A. M., Cole, R. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+with+a+liquid+surface+studied+by+the+aid+of+instantaneous+photography.+Paper+II.">Impact with a liquid surface studied by the aid of instantaneous photography. Paper II.</a> Philosophical Transactions of the Royal Society of London. , 175 (1900).</li><li>Duez, C., Ybert, C., Clanet, C., Bocquet, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Making+a+splash+with+water+repellency.">Making a splash with water repellency.</a> Nature Physics. 3, 180-183 (2007).</li><li>Tan, B. C. W., Thomas, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+an+upper+layer+liquid+on+the+phenomena+and+cavity+formation+associated+with+the+entry+of+solid+spheres+into+a+stratified+two-layer+system+of+immiscible+liquids.">Influence of an upper layer liquid on the phenomena and cavity formation associated with the entry of solid spheres into a stratified two-layer system of immiscible liquids.</a> Physics of Fluids. 30, 064104 (2018).</li><li>Shin, J., McMahon, T. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+tuning+of+a+splash.">The tuning of a splash.</a> Physics of Fluids. 2, 1312-1317 (1990).</li><li>Krishnan, S. R., Seelamantula, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+selection+of+optimum+Savitzky-Golay+filters.">On the selection of optimum Savitzky-Golay filters.</a> IEEE Transactions on Signal Processing. 61, 380-391 (2013).</li><li>Cheny, J., Walters, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Extravagant+viscoelastic+effects+in+the+Worthington+jet+experiment.">Extravagant viscoelastic effects in the Worthington jet experiment.</a> Journal of Non-Newtonian Fluid Mechanics. 67, 125-135 (1996).</li><li>Castillo-Orozco, E., Davanlou, A., Choudhur, P. K., Kumar, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Droplet+impact+on+deep+liquid+pools:+Rayleigh+jet+to+formation+of+secondary+droplets.">Droplet impact on deep liquid pools: Rayleigh jet to formation of secondary droplets.</a> Physical Review E. 92, (2015).</li><li>Aristoff, J. M., Truscott, T. T., Techet, A. H., Bush, J. W. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+water+entry+cavity+formed+by+low+bond+number+impacts.">The water entry cavity formed by low bond number impacts.</a> Physics of Fluids. 20, 091111 (2008).</li><li>Aristoff, J., Bush, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Water+entry+of+small+hydrophobic+spheres.">Water entry of small hydrophobic spheres.</a> Journal of Fluid Mechanics. 619, 45-78 (2009).</li><li>Aristoff, J., Truscott, T., Techet, A., Bush, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+water+entry+of+decelerating+spheres.">The water entry of decelerating spheres.</a> Physics of Fluids. 22, (2010).</li><li>Truscott, T., Epps, B., Techet, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unsteady+forces+on+spheres+during+free-surface+water+entry.">Unsteady forces on spheres during free-surface water entry.</a> Journal of Fluid Mechanics. 704, 173-210 (2012).</li><li>Truscott, T. T., Epps, B. P., Belden, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Water+entry+of+projectiles.">Water entry of projectiles.</a> Annual Review of Fluid Mechanics. 46, 355-378 (2013).</li><li>Gekle, S., Gordillo, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generation+and+breakup+of+Worthington+jets+after+cavity+collapse+part+1.">Generation and breakup of Worthington jets after cavity collapse part 1.</a> Journal of Fluid Mechanics. 663, 293-330 (2010).</li><li>Cross, R., Lindsey, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+the+drag+force+on+a+falling+ball.">Measuring the drag force on a falling ball.</a> The Physics Teacher. 169, (2014).</li><li>Cross, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vertical+impact+of+a+sphere+falling+into+water.">Vertical impact of a sphere falling into water.</a> The Physics Teacher. , 153 (2016).</li><li>Dickerson, A. K., Shankles, P., Madhavan, N., Hu, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mosquitoes+survive+raindrop+collisions+by+virtue+of+their+low+mass.">Mosquitoes survive raindrop collisions by virtue of their low mass.</a> Proceedings of the National Academy of Sciences. 109 (25), 9822-9827 (2012).</li><li>Dickerson, A. K., Shankles, P., Hu, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Raindrops+push+and+splash+flying+insects.">Raindrops push and splash flying insects.</a> Physics of Fluids. 26, 02710 (2014).</li></ol>

The authors have nothing to disclose.

This protocol describes the experimental design and best practices for investigations of free-falling spheres onto a deep liquid pool. We begin by highlighting steps necessary for configuring the experiment for vertical impacts. It is important to create an ideal splash environment with the use of a sufficiently large splash zone such that wall effects are negligible9, and a suitable visual scale for extracting kinematics12,13,<...

The characterization of splash dynamics arising from vertical impacts of solid objects on a deep liquid pool1 is applicable to military, naval and industrial applications such as ballistic missile water entry and sea surface landing2,3,4,5. The first studies of water entry were conducted well more than a century ago6,7. Here, we establish clear in-depth protocols and best practices for achieving consistent results for water entry investigations. To aid valid experimental design, a method is presented for the maintenance of sanitary conditions, alteration of interfacial conditions, control of dimensionless parameters, chemical modification of impactor surface, and visualization of splash kinematics.
Vertical impacts of free-falling hydrophilic spheres on the quiescent fluid show no sign of air-entrapment at low velocities8. We find that the placement of thin penetrable fabrics atop the fluid surface causes cavity formation due to forced flow separation1. A meager amount of fabric on the surface amplifies splashing across a range of moderate Weber numbers while sufficient layering attenuates splashing as spheres overcome drag at fluid entry1. In this article, we explain protocols suitable for establishing the effects of material strength on the water entry of hydrophilic spheres.
Cavity forming splashes from hydrophobic impactors show the ascension of a well-developed splash crown, followed by the protrusion of the primary jet high above the surface when compared to their water-liking counterparts8. Here, we present an approach for achieving water repellency through chemically modifying the surface of hydrophilic spheres.
With the advent of high-speed cameras, splash visualization and characterization have become more attainable. Even so, established standards in the field call for the use of a single camera orthogonal to the primary axis of travel. We show that the use of an additional high-speed camera for overhead views is necessary to adjudge spheres strike the intended location.

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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 established protocols allow for the observance of the Worthington jets arising from vertical impacts over a range of Weber numbers <img alt="Equation 5" src="/files/ftp_upload/59300/59300eq5.jpg" /> as seen in Figure 2c. These results are published in Watson et al.1, which can be referenced for the exact experimental conditions used to produce the data presented herein. We focus on the narrow elongated film of fluid protruding abo...

Watch this Scientific Journal Video about Impacts of Free-falling Spheres on a Deep Liquid Pool with Altered Fluid and Impactor Surface Conditions at JoVE.com

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

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 ...

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6.4K Views.  University of Central Florida. 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.

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

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids&#160;and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.
The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.1&#160;Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic ...

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 ...

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 ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Development of an Experimental Setup for the Measurement of the Coefficient of Restitution under Vacuum Conditions

The Discrete Element Method is used for the simulation of particulate systems to describe and analyze them, to predict and afterwards optimize their behavior for single stages of a process or even an entire process. For the simulation with occurring particle-particle and particle-wall contacts, the value of the coefficient of restitution is required. It can be determined experimentally. The coefficient of restitution depends on several parameters like the impact velocity. Especially for fine particles the impact velocity depends on the air pressure and under atmospheric pressure high impact velocities cannot be reached. For this, a new experimental setup for free-fall tests under vacuum conditions is developed. The coefficient of restitution is determined with the impact and rebound velocity which are detected by a high-speed camera. To not hinder the view, the vacuum chamber is made of glass. Also a new release mechanism to drop one single particle under vacuum conditions is constructed. Due to that, all properties of the particle can be characterized beforehand.

The coefficient of restitution is a parameter that describes the loss of kinetic energy during collision. Here, a free-fall setup under vacuum conditions is developed to be able to determine the coefficient of restitution parameter for particles in micrometer range with high impact velocities.

The Discrete Element Method is used for the simulation of particulate systems to describe and analyze them, to predict and afterwards optimize their ...

An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids

An analog, macroscopic method for studying molecular-scale hydrodynamic processes in dense gases and liquids is described. The technique applies a standard fluid dynamic diagnostic, particle image velocimetry (PIV), to measure: i) velocities of individual particles (grains), extant on short, grain-collision time-scales, ii) velocities of systems of particles, on both short collision-time- and long, continuum-flow-time-scales, iii) collective hydrodynamic modes known to exist in dense molecular fluids, and iv) short- and long-time-scale velocity autocorrelation functions, central to understanding particle-scale dynamics in strongly interacting, dense fluid systems. The basic system is composed of an imaging system, light source, vibrational sensors, vibrational system with a known media, and PIV and analysis software. Required experimental measurements and an outline of the theoretical tools needed when using the analog technique to study molecular-scale hydrodynamic processes are highlighted. The proposed technique provides a relatively straightforward alternative to photonic and neutron beam scattering methods traditionally used in molecular hydrodynamic studies.

An experimentally accessible analog method for studying molecular hydrodynamic processes in dense fluids is presented. The technique uses particle image velocimetry of vibrated, high-restitution grain piles and allows direct, macroscopic observation of dynamical processes known and predicted to exist in strongly interacting, high density gases and liquids.

An analog, macroscopic method for studying molecular-scale hydrodynamic processes in dense gases and liquids is described. The technique applies a ...

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 ...

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 ...

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 ...

Buoyancy and Drag on Immersed Bodies

Source: Alexander S Rattner and Sanjay Adhikari; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA
Objects, vehicles, and organisms immersed in fluid mediums experience forces from the surrounding fluid in the form of buoyancy- a vertical upward force due to fluid weight, drag- a resistive force opposite the direction of motion, and lift- a force perpendicular to the direction of motion. Prediction and characterization of these forces is critical to engineering vehicles and understanding the motion of swimming and flying organisms.
In this experiment, the balance of buoyancy, weight, and drag forces on submerged bodies will be investigated by tracking the rise velocity of air bubbles and oil droplets in a glycerin medium. The resulting drag coefficients at terminal rise velocities will be compared with theoretical values.

Source:  Alexander S Rattner and Sanjay Adhikari; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, 
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