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

Summary

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.

Abstract

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 µm/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.

Introduction

Drop impact onto a solid surface is a key process in many applications involving electronic fabrication¹, spray coating², and additive manufacturing using inkjet printing^3,4, 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 µsec) 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 de....

Protocol

1. Fast Imaging Setup (See Figure 1)

1. 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)±0.15 m/sec.
2. Construct and mount a frame to hold the horizontal impact plane, typically a glass plate, under which an inclined reflective mirror is positioned for visualizing the drop impact from the bottom.
3. Place a clean and smooth glass plate onto the h....

Results

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±0.12 m/sec and radius R₀ = 6.25±0.10 mm were generated. The left column shows the impact of an air-oxidized eGaIn drop not prewashed in acid........

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Gallium-Indium Eutectic	Sigma Aldrich	495425-25G
Hydrochloric Acid	Sigma Aldrich	320331-2.5L
Zirconium oxide	Glen Mills Inc.	7200
Phantom V12 and V7 Fast Ccamera	Vision Research	N/A
105 mm Micro-Nikon	Nikon	N/A
12 V / 200 W light Source	Dedolight	N/A
Syringe Pump	Razel	MODEL R9-9E