Microfluidics suggest hydrophilic surfaces retain more oil than hydrophobic ones for groundwater remediation
Sadie Harley
scientific editor
Robert Egan
associate editor
Dr. Seunghak Lee, Jaeshik Chung, and Sang Hyun Kim of the Water Resources Cycle Research Center at the Korea Institute of Science and Technology (KIST) observed how oil and water interact in porous media under various conditions using a microfluidic system that allows precise observation of microscopic fluid flows.
They conducted experiments under constant pressure differential conditions similar to real groundwater flow, and found that oil easily escaped from hydrophobic surfaces, while more oil was retained on hydrophilic surfaces. These observations were verified with an immiscible displacement analytical model.
The work is in the journal npj Clean Water.
A common method for separating oil from water has been to use hydrophobic materials that adsorb oil. What the researchers found through close observation was the opposite of what they had expected. In conditions of flowing through a porous medium, such as groundwater, they found that hydrophilic surfaces—those that bind easily with water molecules—hold onto more oil.
In hydrophobic materials, the contact angle at the interface where water repels oil is larger than in hydrophilic media. This reduces the capillary pressure drop at the interface, but increases the pressure difference due to fluid viscosity, which in turn increases the velocity of the fluid in the pores. This accelerated flow of water in the pores causes more oil to spill out.
In hydrophilic materials, on the other hand, the oil is not pushed out as well by the relatively low flow velocity in the pores under the same pressure conditions, resulting in a large amount of oil remaining.
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Illustration and analytical model describing the composition of the pressure drops that occur when two fluids (permeate and conventional) move through a capillary. Demonstrates that the flow of fluids is determined by the interaction of capillary pressure and viscous pressure, rather than simply by viscosity or pressure difference. Credit: Korea Institute of Science and Technology (KIST) -
Extending experimental results at the pore scale to a more skin-touching continuum scale with practical environmental implications. In this study, we show that this can lead to higher flow rates and less residual oil in hydrophobic environments. This suggests that the conventional conception that "hydrophobicity holds oil" may be different under background fluid flow caused by pressure differences, such as in groundwater environments. Credit: Korea Institute of Science and Technology (KIST)
This study goes beyond simple fluid behavior analysis and provides a new interpretive framework for the migration and settling of contaminants in groundwater. The results are expected to contribute to the effective design and operation of pollution prevention facilities such as Permeable Reactive Barrier (PRB) to control oil pollution in groundwater, which is common at military bases and gas station sites.
"Groundwater remediation is not just a matter of materials science, but a representative multiphysics phenomenon that involves a complex interplay of fluid flow and interfacial reactions," said Dr. Jaeshik Chung, KIST.
"This research can be applied not only to groundwater remediation, but also to various immiscible displacement processes in porous media, such as enhanced oil recovery (EOR) and carbon capture and storage (CCS)."
"This achievement shows that underground fluid flow can behave completely differently from existing scientific theories under certain conditions," said Dr. Seunghak Lee of KIST, adding, "This research lays the scientific foundation for more precise control of the underground environment."
More information: Kyung-Jin Lee et al, Revisiting hydrophobicity and its effectiveness in oil retention using microfluidic experiments, npj Clean Water (2025).
Journal information: npj Clean Water
