Materials that repel water are used in countless applications, including industrial separation processes, routine laboratory pipetting, and medical devices. When water touches these surfaces, the interface where they meet tends to acquire a small electrical charge — an effect that is ubiquitous, yet poorly understood.
KAUST researchers have now studied this in detail and their findings could have broad implications [1] .
“This is not a niche laboratory curiosity,” says Yinfeng Xu, a Ph.D. student who led the experimental work in Himanshu Mishra’s laboratory. “This phenomenon plays a role in environmental processes such as dew droplets and raindrops; in industrial operations involving sprays, condensates, or emulsions; and in modern microfluidic and liquid-handling systems used in laboratories worldwide.”
Xu conducted a series of experiments that involved drawing water into a hydrophobic capillary tube, and then dispensing droplets into a Faraday cup — a copper container connected to a sensitive electrometer that measured the tiny electrical charge carried by the droplets. Hydrophobicity ensured that water left the capillary as a drop, without leaving much trace behind.
The researchers used glass capillaries with different chemical coatings to understand if the charging effect was due to water or the surface. This revealed that the droplets could carry either positive or negative charge, depending on the coating.
Next, Xu varied how quickly water was taken up and released. Surprisingly, he found that the rate of liquid uptake had almost no effect on the charge, and neither did the length of time that the water remained in the capillary. In contrast, the rate of droplet release had a significant impact: “In simple terms, the faster the liquid pulls away from the surface, the more charge is generated,” says Mishra.
The team then studied the effect of repeatedly drawing water into a capillary and releasing droplets into the Faraday cup. This confirmed that the final step of each cycle — droplet release — was where most of the charging occurred. “One would intuitively expect that charging happens mainly when water first touches a surface,” says Mishra. “Our findings show that the opposite is true.”
Curiously, changing the rate of droplet release also affected charging during the uptake step of the following cycle. “The interface does not simply return to a neutral state after each droplet,” says Xu. “It ‘remembers’ its recent past, and that memory influences how charge is transferred in subsequent cycles.”
Together, these findings help to reconcile previously reported contradictory observations by other scientists. They also provide a basis for further investigation of exactly how the charging process happens.
Meanwhile, the study has significant implications for industrial processes involving liquids, or for microfluidic devices that handle very small volumes of liquid. “In microfluidic systems, even small amounts of charge can noticeably influence how particles, droplets, or biomolecules move,” explains Xu. “By clarifying when and how charge is generated, our findings can help improve the reliability and reproducibility of microfluidic devices, especially in experiments that rely on precise control of tiny liquid volumes.”
The researchers plan to develop a theoretical model that could predict the generation of charge based on the movement of water across hydrophobic surfaces.
Reference
Langmuir