Everything You Ever Wanted to Know About Droplets

(Image courtesy of Keshavarz et. al.)

A team of engineers has developed a new method for determining the distribution of droplet sizes in splattered liquids. The research could prove useful in a variety of industrial applications, such as helping to prevent defects in automotive paint jobs and optimizing farm fertilization.

Beads on a String

Previous research in this area resulted in an equation to determine the droplet distribution of Newtonian fluids, such as oil and water, which are relatively thin and homogeneous. However, this equation fails to describe non-Newtonian fluids like blood, paint and saliva.

The researchers suspected that the mismatch was due to the viscoelasticity, or stickiness, of non-Newtonian fluids. They set up experiments to observe liquid fragmentation in both types of fluid, with three different atomization tests: first, they dropped the liquids onto a flat surface; second, they sprayed the liquids through a nozzle; and third, they formed a spray by colliding two jets. They observed their experiments with a strobe-light technique to create split-millisecond images of the droplets.

The researchers discovered that, in general, Newtonian fluids produce a narrower range of droplet sizes than non-Newtonian fluids. The more viscoelastic fluids also created long, string-like ligaments before breaking apart into both big and small drops.

Example of a ligament breaking apart into various sizes of droplets. The more viscoelastic the fluid, the bumpier the ligaments. (Image courtesy of Physical Review Letters.)

The researchers discovered several interesting features about the ligaments that form in atomization. For example, the more viscoelastic the liquid, the bumpier the ligament. However, past a certain viscoelasticity, the degree of ligament bumpiness becomes constant. In addition, the time required for the ligament to thin, called the relaxation time, also becomes constant for viscoelastic liquids.

Combining their new research with the previous formula for droplet distribution, the engineers determined a maximum distribution for the droplet sizes of any viscoelastic, non-Newtonian fluid. “Regardless of the type of experiment, or the kind of polymer or concentration, we see this universal distribution, and it’s broadly applicable to a wide range of fluids,” said researcher Gareth McKinley.

Understanding Fluid Fragmentation

The new knowledge about non-Newtonian droplet distribution should prove useful in optimizing any process involving liquid sprays, such as spray painting, inkjets, agricultural sprays or automotive painting.

“When they spray a car, they have to tape the windows because no matter how careful you are, there’s always some overspray, which is wasted paint,” said McKinley. “Also, if you’re spraying paint, the biggest drops tend to show up as defects. That’s one reason you care about droplet size distribution: You want to know how big the biggest drops will be, because a good paint job at the end of the day should be a perfectly smooth finish.”

You can read the team’s full paper in Physical Review Letters. For more fine-tuning news, check out Tiny Squeeze Boosts Performance of Platinum Catalysts.