Academia, Fluid Dynamics | October 6, 2023

Enhance Droplet Impact Research with High-Speed Cameras

Brief history of droplet impact

The study of droplet impact can be traced back to the nineteenth century, with the pioneering work by Worthington[1]. His book, published in the 1890’s, portrays outstanding images of individual droplets[2] after impact with a solid surface. The volume contains images drawn by hand. A variable time delay triggered a short-lived electric spark to illuminate the scene. Hence he could observe a specific shape of the droplet at different stages of the impact process. In later instances he used a photographic camera. This removed the subjectivity of his interpretation when drawing the droplet shape from his recollection.

Worthington noticed a couple of facts. First: “the event takes place in the twinkling of the eye”. Second, he wrote, “it is an exquisite phenomenon that illustrates some of the fundamental properties of fluids”.

Certainly the event is fast, thus many of its features eluded clarification for decades. He was totally right in his second remark. The beauty of this seemingly innocuous process still captivates the eyes of the public nowadays all over the world when presented in slow motion or in still frames.

Little did he know that his work would motivate a large number of scholars to continue investigating this field. In fact, the interest in the subject is still strong nowadays, almost 150 since his seminal work. Interestingly, a significant number of phenomena have been more thoroughly understood only in the last couple of decades due to the advances in high-speed imaging and optical components. High-frame rates coupled with strong optics allow researchers to record finer details of the underlying mechanisms of droplet impact. This has resulted in a surge in publications related to this remarkable theme relevant to large number of fields. In industry, it is related to spray painting, fuel injection, cooling, inkjet printing, 3D printing, biological material deposition, agriculture and power generation, to name a few [3–6].

In nature, the impact of raindrops creates splashing which produces aerosols[4,6]. We all have experienced the smell of fresh soil on a rainy day[7]. Yet, this same mechanism has been associated with foliar disease outbreaks and pathogen dispersion[8].

Enhance your study of droplets with Chronos High-Speed Cameras

A typical droplet impact event is completed in a few milliseconds. Thus, it is especially suited for the frame rates of the Chronos High-Speed Cameras. For instance, if the impact velocity is only a few m/s a high-speed camera recording at 1 to 5 kHz provides enough information to resolve the impact of the droplet against the solid substrate. Also, with a pixel size of the Chronos 1.4 and 2.1-HD cameras, 6.6 and 10 μm respectively, the Chronos cameras provide sufficient spatial resolution when coupled to a macro lens. Figure 1 displays an example of the impact of a coffee droplet with a solid surface. The images resolve the thin film produced as the droplet expands on the solid wall, left image, lower row. Also, the small receding waves moving towards the center of the film, are well resolved in the center image, lower row.

Figure 1. Impact of a coffee droplet on a solid surface

Figure 2 displays the impact of a coffee droplet on a pool of coffee. The initial splashing of tiny droplets is seen in the first image on the left side. The “Worthington jet” that emerges after the cavity opened by the droplet closes is depicted in the central image. The image on the right depicts a satellite droplet about to be detached from the liquid column below. The thickness of the neck joining the satellite droplet and the liquid column is only 260 μm. You can take a look at our gallery of droplet impact to see the beauty created by these rather simple events.