Thanks to Wendy Zhang, Luuk Lubbers, Marc Miskin and Michelle Driscoll for many useful discussions and Qiti Guo for help with preparing experimental samples. This work was supported by the National Science Foundation&#39;s MRSEC program under Grant No. DMR-0820054.

1. Fast Imaging Setup (See Figure 1)
<ol>
	<li>Start by setting up a vertical track along which a container filled with the fluid to be studied can be freely moved to adjust the impact velocity. The fluid leaves the bottom of the container through a nozzle and then enters free fall. For this work the falling height was varied from 1-200 cm to give an impact velocity V0 = (0.4-6.3)&#177;0.15 m/sec.</li>
	<li>Construct and mount a frame to hold the horizontal impact plane, typically a glass plate, under which ...

<ol>
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The authors have&#160;nothing to disclose.

Several steps are critical for proper execution of the fast imaging. First, camera and lens have to be appropriately set up and calibrated. In particular, in order to get high spatial resolution, the reproduction ratio of the lens must be kept close to 1:1. This is especially important for the visualization of dense suspensions. Also, the aperture size needs to be carefully chosen for imaging. For instance, observation from the side in general requires a longer depth of field, therefore smaller aperture size. To maintain...

Drop impact onto a solid surface is a key process in many applications involving electronic fabrication1, spray coating2, and additive manufacturing using inkjet printing3,4, where a precise control of drop spreading and splashing is desired. However, direct observation of drop impact is technically challenging for two reasons. First, it is an intricate dynamic process that occurs within a timescale too short (~100 &#181;sec) to be imaged easily by conventional imaging systems, such as optical microscopes and DSLR cameras. Flash photography can of course image much faster, but does not allow for continuous recording, as required for detailed analysis of the evolution with time. Second, the length scale induced by impact instabilities can be as small as 10 &#181;m&#160;5. Therefore, to quantitatively study the impact process a system that combines ultrafast imaging along with reasonably high spatial resolution is often desired. In the absence of such system, early work on droplet impact focused mostly on the global geometric deformation after impact6-8, but was unable to gather information about the early time, nonequilibrium processes associated with impact, such as the onset of splashing. Recent advances in CMOS high speed videography of fluids9,12 have pushed the frame rate up to one million fps and exposure times down below 1 &#181;sec. Furthermore, newly developed CCD imaging techniques can push the frame rate well above one million fps9-12. Spatial resolution on the other hand, can be increased to the order of 1 &#181;m/pixel using magnifying lenses12. As a consequence, it has become possible to explore in unprecedented detail the influence of a wide range of physical parameters on various stages of drop impact and to systematically compare experiment and theory5,13-16.&#160;For instance, the splashing transition in Newtonian fluids was found to be set by atmosphere pressure5,&#160;while the intrinsic rheology decides the spreading dynamics of yield-stress fluids17.
Here a simple yet powerful fast imaging technique is introduced and applied to study the impact dynamics of two types of non-Newtonian fluids: liquid metals and densely packed suspensions. With exposure to air, essentially all liquid metals (except mercury) will spontaneously develop an oxide skin on their surface. Mechanically, the skin is found to alter effective surface tension and wetting ability of the metals18.&#160;In a previous paper15, several of the authors studied the spreading process quantitatively and were able to explain how the skin effect influences the impact dynamics, especially the scaling of the maximum spreading radius with impact parameters. Since liquid metal has high surface reflectivity, careful adjustment of the lighting is required in the imaging. Suspensions are composed of small particles in a liquid. Even for simple Newtonian liquids, the addition of particles results in non-Newtonian behavior, which becomes especially pronounced in dense suspensions, i.e.&#160;at high volume fraction of suspended particles. Particularly, the onset of splashing when a suspension droplet hits a smooth, hard surface was studied in the previous work16.&#160;Both liquid-particle and inter-particle interactions can change the splashing behavior significantly from what might be expected from simple liquids. To track particles as small as 80 &#181;m in these experiments a high spatial resolution is needed.
A combination of various technical requirements such as high temporal and spatial resolution, plus the capability for observing impacts both from the side and from below, can all be satisfied with the imaging setup described here. By following a standard protocol, described below, the impact dynamics can be investigated in a controlled fashion, as shown explicitly for spreading and splashing behavior.

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		<col />
		<col />
		<col />
		<col />
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	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Name of Material/ Equipment</td>
			<td style="width:103px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments/Description</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Gallium-Indium Eutectic</td>
			<td>Sigma Aldrich</td>
			<td>495425-25G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Hydrochloric Acid&#160;</td>
			<td>Sigma Aldrich</td>
			<td>320331-2.5L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Zirconium oxide</td>
			<td>Glen Mills Inc.</td>
			<td>7200</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Phantom V12 &#38; V7 Fast Ccamera</td>
			<td>Vision Research</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">105mm Micro-Nikon</td>
			<td>Nikon</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">12V/200W light Source</td>
			<td>Dedolight</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Syringe Pump</td>
			<td>RAZEL</td>
			<td>MODEL R9-9E</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

The fast imaging technique can be used to quantify spreading and splashing for various impact scenarios. Figure 4(a), for instance, shows typical impact image sequences for liquid eGaIn with different oxide skin strength. By ejecting eGaIn from the same nozzle and at the same falling height, droplets with reproducible impact velocity V0 = 1.02&#177;0.12&#160;m/sec&#160;and radius R0 = 6.25&#177;0.10&#160;mm&#160;were generated. The left column shows the impact of an air-oxidized eGa...

Watch this Scientific Journal Video about Fast Imaging Technique to Study Drop Impact Dynamics of Non-Newtonian Fluids at JoVE.com

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

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 ...

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