A high-speed camera captures how a water jet pierces a droplet

The results may help engineers develop a way to inject drugs without needles.


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In a new study by MIT and the University of Twente in the Netherlands, scientists fired small water jets through droplets. They did so to see the drop impact as the water jet pierced through the droplet. Using high-speed cameras, scientists captured a water jet’s splashy impact as it pierces a droplet.

As the droplets in their experiments are transparent, the scientists could also track what happens inside a droplet as a jet is fired through.

This experiment reminds the famous strobe-light photographs of a bullet piercing an apple, pioneered by MIT’s Harold “Doc” Edgerton. Edgerton, in that study, captured sequential images of a bullet being shot through an apple in unprecedented detail. The MIT team’s new videos of a water jet fired through a droplet reveal surprisingly similar impact dynamics.

The experiment’s outcomes led scientists to develop a model to predict how a fluid jet will impact a droplet of a specific viscosity and elasticity. What’s more, it could help them identify approaches to inject fluids, such as vaccines through the skin without using needles.

David Fernandez Rivas, a research affiliate at MIT and professor at the University of Twente, said, “We want to explore how needle-free injection can be done in a way that minimizes damage to the skin. With these experiments, we are getting all this knowledge to inform how we can create jets with the right velocity and shape to inject into the skin.”

The properties of water are better known and can be carefully calibrated. Hence decided to study in detail a simpler injection scenario: a jet of water fired into a suspended droplet of water.

For the study, scientists set up a laser-based microfluidic system. They fired thin jets of water at a single water droplet or pendant -hanging from a vertical syringe.

They varied the viscosity of each pendant by adding certain additives to make it as thin as water or thick like honey. They then recorded each experiment with high-speed cameras.

Scientists also measured the speed and size of the liquid jet that punctured and sometimes pierced straight through the pendant. The examinations uncovered fascinating phenomena, for example, instances when a jet was dragged back into a pendant because of the pendant’s viscoelasticity. As the jet pierced the pendent, it generated air bubbles as well.

Rivas said, “Understanding these phenomena is important because if we are injecting into the skin in this way, we want to avoid, say, bringing air bubbles into the body.”

The video revealed the jet’s shape as it penetrated through the pendant. Scientists found that the jet’s shape is more complex than a simple cylinder. Hence, scientists decided to develop another model based on a known equation by physicist Lord Rayleigh. The Rayleigh equation describes how the shape of a cavity changes as it moves through a liquid.

They modified the equation to apply to a liquid jet moving through a liquid droplet and found that this second model produced a more accurate representation of what they observed.

Ian Hunter from MIT said“This new method of generating high-velocity microdroplets is essential to the future of needle-free drug delivery. An understanding of how these very fast-moving microdroplets interact with stationary liquids of different viscosities is an essential first step to modeling their interaction with a wide range of tissue types.”

Journal Reference:

  1. Miguel A. Quetzeri-Santiago et al. Impact of a microfluidic jet on a pendant drop. DOI: 10.1039/D1SM00706H