Tears of an evaporating meniscus illustrates a new method for the generation of tiny droplets

Purdue researchers have observed a new phenomenon in fluid mechanics, involving the three-phase contact line where a solid, liquid, and gas phase meet. By observing methanol as it evaporates, they identified "tears" forming at regular intervals.

“We are focusing on the instabilities of the line where solid, liquid, and air intersect,” said Monojit Chakraborty, post-doctoral research associate working with professors Suresh Garimella and Justin Weibel.  “The formation of droplets due to instabilities is commonly observed in nature, such as when liquids form patterns during the splashing of raindrops, or when a stream of liquid breaks up into droplets when we slowly open a tap.  But formation of tiny droplets using this particular instability has never been demonstrated before.”

The experiment involved depositing a drop of methanol on a silicon wafer, enough to form a thin film of liquid on the surface.  As the methanol evaporated, the contact line at the edge of the liquid film retracted quickly, leaving behind a series of tear-like drops that are consistently equidistant, and contain roughly the same volume of liquid.

“Using high-speed cameras, we can observe exactly what’s happening as this line retracts,” said Chakraborty.  “A rim of liquid forms at the contact line, which begins to undulate with a constant specific frequency.  At the peak of the undulation, a droplet forms and is left behind. Then the rim builds up again, leaving another droplet along that same predictable pattern.”

The rim formation is caused by “autophobicity,” when liquid molecules adhering to a solid surface repel other adjacent molecules in the liquid.

The research was recently showcased as an Editor's Pick by Applied Physics Letters.  It took place at the Cooling Technologies Research Center at Purdue's School of Mechanical Engineering.

Tears of an evaporating methanol meniscus on a silicon substrate
Monojit Chakraborty, Justin A. Weibel, and Suresh V. Garimella


We identify the formation of an unstable rim at the three-phase contact line of an evaporating methanol meniscus due to the autophobic nature of the liquid. Undulations along the rim rupture at a consistent frequency, leaving behind a regular pattern of droplet tears as the contact line recedes. We characterize the geometry and undulation dynamics of the rim. The rim movement exhibits slip at the contact line; the measured ratio of rim width to undulation wavelength matches the critical unstable Rayleigh ratio for an intermediate slip regime. Unlike previously observed rim instabilities, the rim volume here is replenished from the evaporating bulk meniscus and maintains a constant width, such that the instability wavelength remains constant and droplets are generated with a consistent, nano-liter volume and deposited in a regular pattern. Occurrence of this unstable rim during an evaporation-driven dewetting process may allow for rapid, controlled deposition of small droplets from the fast-moving contact line of a highly volatile fluid.