Vibration is one method for breaking a drop into smaller droplets, a process known as atomization. Here, researchers simulate this break-up process for a drop in microgravity. Waves crisscrossing the surface create localized craters and jets, making the drop resemble the Greek mythological figure of Medusa. With enough vibrational amplitude, the jets stretch to point of breaking, releasing daughter droplets. (Image and research credit: D. Panda et al.)
Search results for: “droplet”
Linking Size and Origin in Droplets
Respiratory diseases like measles, flu, tuberculosis, and COVID-19 are all transmitted by droplets. Some are tiny and airborne, capable of traveling long distances. Other drops are larger and only capable of traveling short distances. A new review paper consolidates what we know about these droplets and categorizes them by size and origin.
It turns out that a droplet’s size can tell us where it originated in the body. The largest type of droplets come from our mouths, lips, and tongues. Some form from filaments of saliva that stretch across our mouths and burst during exhalation. Others originate in our nasal passages where a sneeze can destabilize the mucus film there. These types of droplets are best suited to transmitting diseases that reside in the upper respiratory tract. Coughing, sneezing, singing, and speaking all produce these droplets, but breathing does not.
In contrast, the smallest classes of droplets come from the bronchial passages of the lungs, where films form after exhalation closes a passage. When we inhale again, the passage reopens, the film breaks up, and tiny droplets flow further into the lungs before getting exhaled. Breathing alone is enough to create and spread these tiny droplets, which are well-suited to spreading diseases that reside deep in the lungs, like tuberculosis.
In between these extremes are medium-sized droplets created from movement around our vocal cords. The formation mechanism for these droplets is least understood, but they are connected to breathing, coughing, speaking, singing, and so on.
Ultimately, understanding the mechanics of disease transmission is about knowing how to best prevent transmission. Knowing the size of droplets responsible for transmission lets us prioritize responses that work. For example, if large droplets are the primary transmission mechanism, loose-fitting masks and face masks will stop the spread. But for smaller droplets, ventilation measures and well-fitted N-95 respirators are the better choice. (Image credit: Anton; research credit: M. Pöhlker et al.; via APS Physics)
Scooting Droplets
As a child, I always loved watching rain on the windows as I rode in the car. Hemispherical droplets got stretched by the wind flowing over them. But they never stretched smoothly; instead they seemed to shiver and shake unevenly. A recent study looks at a similar situation: drops of glycerin forced to slide along a horizontal surface under the force of the wind. Like the drops on my parents’ car, the glycerin gets stretched out into an elongated oval. Surface waves develop atop the drop and move downstream. The drops, the authors observe, move a bit like a crawling caterpillar, pilling up and smoothing out as they move. (Image credit: rain – A. Alves, experiment – A. Chahine et al.; research credit: A. Chahine et al.; via APS Physics)
This series of images shows an elongated droplet subjected to airflow moving from left to right. Waves form on the drop and move downstream in a fashion similar to a caterpillar crawling. 
Gravity Changes Droplet Shapes
With small droplets, gravity usually has little effect compared to surface tension. An evaporating water droplet holds its spherical shape as it evaporates. But the story is different when you add proteins to the droplet, as seen in this recent study.
The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens. As a protein-doped droplet sitting on a surface evaporates, it starts out spherical, like its protein-free cousin. But, as the water evaporates, it leaves proteins behind, gradually increasing their concentration. Eventually, they form an elastic skin covering the drop. As water continues to evaporate, the droplet flattens.
For a hanging droplet, the shape again starts out spherical. But as the drop’s water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles. In contrast, a hanging droplet with proteins takes on a wrinkled appearance once its elastic skin forms. The key difference, according to the model constructed by the authors, is the direction that gravity points. Despite these droplets’ small size, gravity makes a difference! (Image, video, and research credit: D. Riccobelli et al.; via APS Physics)
Giant Droplet Splashes
When droplets get larger than 0.27 cm, they no longer stay spherical as they fall. Here, researchers look at very large droplets (equivalent to 3.06 cm in diameter) falling into water. On their way to the pool, the droplets oscillate — some lengthening, some flattening, and some bulging into a bag. The droplet’s shape at impact (and its speed) determine what shape of splash and cavity form. Wider drops make wider and shallower cavities. (Image credit: S. Dighe et al.)
Self-Propelled Droplets
Drops of ethanol on a heated surface contract and self-propel as they evaporate. My first thought upon seeing this was of Leidenfrost drops, but the surface is not nearly hot enough for that effect. Instead, it’s significantly below ethanol’s boiling point. Looking at the drops in infrared reveals beautiful, shifting patterns of convection cells on the drop. The patterns are driven by the temperature difference along the drop; at the bottom, the drop is warmest, and at its apex, it is coldest. Those differences in temperature create differences in surface tension, which drives a surface flow that breaks the drop’s symmetry. The asymmetry, the authors suggest, is responsible for the drop’s propulsion. (Image and video credit: N. Kim et al.)
DIY Superwalking Droplets
Over the past few years, we’ve seen lots of research in walking droplets, especially as hydrodynamic quantum analogs. But did you know you can replicate this set-up at home and play with it yourself? This video gives an overview of the equipment you’ll need and a simple procedure to follow to get it up and running. From there, your imagination is the limit! (Image and video credit: R. Valani)
Droplet Bounce
A droplet falling on a liquid bath may, if slow enough, rebound off the surface. Its impact sends out a string of ripples — capillary waves — on the bath’s surface and sends the droplet itself into jiggling paroxysms. A new pre-print study delves into this process through a combination of experiment, simulation, and modeling. Impressively, they find that the most of the droplet’s initial energy is not dissipated during impact. Instead it’s fed into the capillary waves and droplet deformation that follow. (Image and research credit: L. Alventosa et al.; via Dan H.)
A droplet falls on a bath, partially coalesces and rebounds. The process repeats until the droplet is small enough to coalesce completely. The Yarning Droplet
Marangoni bursting takes place in alcohol-water droplets; as the alcohol evaporates, surface tension changes across the liquid surface, generating a flow that tears the original drop into smaller droplets. Here researchers add a twist to the experiment using PMMA, an additive that dissolves well in alcohol but poorly in water. As the alcohol evaporates, the PMMA precipitates back out of the water-rich droplet, forming yarn-like strands. (Image and video credit: C. Seyfert and A. Marin)
