Outline.

Combining experimental studies, micro/nanofabrication, and theoretical analysis this project has examined the feasibility of an innovative approach based on capillary action for the separation of water and oils, generally defined as organic compounds insoluble in water. We have designed, fabricated, and tested simple fluidic devices called capillary diodes (Figure 1) for potential application in passive separation and filtering processes for immiscible liquids in water. The concept is analogous to conventional diodes, since a junction between two surfaces with dissimilar surface energies induce a threshold pressure head that must be overcome to observe the ?forward? transport of certain liquid pairs. In vertical imbibtion, the threshold pressure head corresponds to a critical immersion depth of the device inlet (Figure 1).

Intellectual merit

By designing the surface micro/nanostructure and geometry of the capillary conducts with section having dissimilar surface energies we proved that one can effectively promote or prevent the spontaneous (capillarity driven) imbibition of selected fluid pairs (e.g., water-air, oil-air, water-oil). Our work has been able to predict, induce, and control the effective hydrodynamic conductivity and the threshold pressure head to observe imbibition (forward liquid displacement) and drainage (backward liquid displacement) of water and oils in a rectangular glass capillary that was partially micro/nanotextured using a single-step laser-based fabrication method. The fabrication approach developed and employed as a part of this project produces both a well-defined micropattern with controlled geometry and the deposition of nanomaterial with complex morphologies (Figure 2). The nanomaterial deposition and coverage extent was a critical factor to control the surface energy and resulting affinity of different liquids to each section the device surface.

 The key hypothesis that this project examined and verified is that significant asymmetries in the wetting and de-wetting dynamics by specific fluid pairs (water-air, oil-air, oil-water) can be effectively controlled through the surface micro/nanostructured in order to engineered capillary "diodes" for potential applications as passive separators and filters for immiscible organic liquids in water. To test or working hypothesis, we employed a laser-based method to design and fabricate surfaces having microscale patterns and complex nanoscale structures that once infused by water or oil break the symmetry of the energies and forces required for wetting/de-wetting and the associated forward/backward fluid transport inside the device. This resulted in the ability to prevent the spontaneous imbibition and wetting by simple oils when the micro/nanopatterned slits were pre-infused by water and immersed in water-oil mixtures at controlled immersion depths that were predicted by analytical models (Figure 3).

Broader impacts

The research work in this project has contributed to the fundamental knowledge of (dynamic) wetting/dewetting processes and liquid adhesion phenomena on surfaces having complex micro/ nanostructure. A better understanding of these phenomena is relevant to a wide range of natural processes and industrial applications. The new knowledge gained and the predictive models produced in this project can help develop new strategies for anti-fouling, drag reduction, and micro/nano-fluidic handling in diverse engineering applications. The theoretical developments and experimental analysis produced by this project indicate that the dynamics of imbibition and drainage processes present nontrivial regimes with long-lived metastable states that are not described by conventional continuum-based predictions (e.g., the Lucas-Washburn equation). To understand the presence of metastable states and the hindering of imbibition or drainage by different liquid pairs required new analytical descriptions considering the effects of thermal motion and the surface nanostructure, which largely dominate the wetting dynamics near thermodynamic equilibrium conditions. This project has thus generated fundamental knowledge and analytical tools to help understand and control the dynamics of imbibition and drainage in synthetic nano/microstructured materials such as filters, sponges, and fabrics, or natural porous media that are relevant to environmental engineering applications such as soil, sandstone, or biological membranes.

This project supported 3 PhD students who have graduated during the completion of the project, 2 of these students belong to underrepresented minorities in STEM.

Last Modified: 06/09/2021
Modified by: Carlos E Colosqui