Extracting the 3D structure of liquid surfaces

Published 27 January 2023

On the left are the images used to 3D reconstruct the liquid surface shown on the right.

When you are cleaning your car with your high-pressure washer have you ever considered how it works? Well we have and to figure that out we need to see how the liquid looks like in 3D. As is seen in the video above we have 3D reconstructed the surface of a cone-like liquid using a technique developed at the division of combustion physics. This technique apply lasers and a single high-speed camera to capture a full event of what is going on in the liquid. With this new 3D information it is possible to get a deeper understanding of the forces that shape and evolve the liquid that which important for many different applications.

Liquids and more specifically sprays are widely used in our society. To mention a few applications, sprays are used in inhalers to dose medicine, they are used to create milk powder, they are used in combustion engines and in painting. Each of these specific applications have different requirements on the sprays where the parameters that creates the spray is fine tuned. The so called injector is one of the main parameters that simplified can be explained as a small hole that you push liquid through. The liquid will then spread out and be divided into a cloud of small drops after escaping the small hole. This cloud is what we call a spray. Here, the type of liquid and the injection pressure (how hard we push the liquid through the hole) are also important in understanding how the spray will be formed. In our work at combustion physics, we are creating measurement techniques that makes it possible to measure new things in sprays and liquids. This work is about technique that we have developed to 3D reconstruct liquid surfaces.

To reconstruct 3D coordinates you typically have more than one camera or you move the camera so that the object to be 3D reconstructed are imaged from different views. But this is not always possible. To move the camera when we have a spray is hard since the spray is changing shape and changing it fast. To capture the changes we are here recording with 20 000 frames per second. This is so fast that if we were to film you when you blink, we would need to record at least 2000 frames to capture it. In other words, there is no time to move the camera since then the liquid is changing must faster.

If moving the camera, does not work to get 3D, then you can use multiple cameras, right? Yes that is true, however, the camera that can perform this high frame rate that we require is quite expensive (so expensive that they are not listing the price on their webpage, see link below) which means that we would prefer to only have one. If we only have one camera, information of 3D must come from something else than what is typically done. We do it by using laser lines. If you look at the video above when paused you can see that there are multiple lines curving along the surface of the liquid. What we are projecting onto the surface are actually straight lines (called fringes). The 3D curvature of the surface of the liquid is what makes the lines curve. By extracting this curvature we can reconstruct the surface shape as is shown in the video.

So then we have 3D, but there is one final important thing that we do to make this work. There is a typical problem when shining a laser or any light for that matter on liquid. The liquid will reflect the light back brightly in some areas and almost not at all in other. This is problematic when we want see these small lines all over the liquid. To solve this we use fluorescence. After adding a fluorescent dye to the liquid, it will glow in a different color than the laser color that we are shining onto it. This glowing will be much more smooth than the reflections. By then using a filter that only lets this fluorescence color through we can remove all light with the color of the laser that is irregular and keep the smooth fluorescence glow. Now the lines are visible and the curvature can be used. We call this technique Fringe Projection - Laser Induced Fluorescence.

The Fringe Projection - Laser Induced Fluorescence technique require only one of these expensive camera, which means that it is accessible to more researchers. In addition, this new 3D information of liquid surfaces takes us one step forward in understanding how to better clean your car with a pressure washer (among other).