The Magic of Carefully Crafting Liquids

Wells of various concentrations of PG (colors) confined by Sharpie lines (black). Droplets move under gravity, sampling each well, but merge only with like concentration. Sorting is passive.

Spontaneous droplet aligner made of green droplets dispensed at random positions that automatically align. Sharpie marker lines are hydrophobic to prevent drops from mixing.Short-range chasing between a 1% PG droplet (red) and a 25% PG droplet (blue) in a circle. Droplet with more water exerts a higher surface tension tug on the other.

Droplets containing two liquids with different properties can be made into fluid machines. Engineers in the lab of Stanford University’s Manu Prakash (creator of the 50-cent paper microscope) found that mixtures of simple ingredients like water and propylene glycol-based food coloring could propel themselves in intricate patterns and move other droplets around a standard glass slide.

“We demonstrate experimentally and analytically that these droplets are stabilized by evaporation-induced surface tension gradients and that they move in response to the vapour emitted by neighbouring droplets,” the authors write in a paper published yesterday in the journal Nature. “Our fundamental understanding of this robust system enabled us to construct a wide variety of autonomous fluidic machines out of everyday materials.”

The work trying to understand the dynamics of these different fluids began with an unexpected observation by coauthor and bioengineering graduate student Nate Cira in 2009. While completing an unrelated experiment as an undergraduate, Cira noticed that drops of food coloring began to move of their own accord on a glass slide. See video below.

Cira took his work figuring out what was happening with the droplets to Prakash’s lab and, after years of study, have come to some interesting realizations.

“These droplets sense one another, they move and interact, almost like living cells,” saidPrakash, an assistant professor of bioengineering.

They write that the droplets each contains one liquid that wets the slide and another that doesn’t. The non-wetting component maintains a contact angle with the surface like those seen in hydrophobic liquids. Meanwhile the wetting component of the drop spreads out underneath to form a thin fluid film. And droplets move and interact with each other because of a constantly shifting fight between surface tension and evaporation happening in each drop. Because water evaporates more quickly than propylene glycol and has higher surface tension, molecular dynamics cause the fluids inside the drop to churn.

Using varying mixtures of the two ingredients, the researchers observed different behaviors and properties. One experiment, for instance, saw one droplet with higher water concentration pulling another along that had less water in it. Read more about how the droplets work here.

The work offers more than just a curiosity, though. They say such engineered drops could be used to make self-assembling liquid lenses that cooperate to produce a focused image. Advances may also be possible in semiconductor manufacturing and developing self-cleaning solar panels.

All gifs created from videos courtesy of Nate Cira, Adrien Benusiglio, Manu Prakash