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Description
In this work, we propose a new method based on in situ plasma treatment to alter the wettability of microfluidics devices. The targeted contact angle remains stable for several days, offering new possibilities for investigating multiphase flow in porous media. Moreover, we demonstrate the influence of wettability alteration on two-phase flow and transport properties.
Microfluidics contributes to a better understanding of the multi-scale and multi-physics processes in geological environments. For example, the success of a secure and permanent storage of CO2 in subsurface formations depends largely on our knowledge of two-phase fluid displacements and interphase mass transfer leading to residual saturation and dissolution of CO2. In these multi-fluid flow problems, a complex interplay of physico-chemical factors controls the fluid displacements, trapping, dissolution, and remobilization. The wettability is one crucial parameter that affects residual trapping and interphase mass transfer. Although the importance of wettability on multiphase flows and mass transfer is recognized, there is a lack of reliable and high-resolution data on its influence.
We developed a new method based on plasma treatment to control the wettability of microfluidic devices. Plasma is a promising tool, yet its propagation in microchannels and the stability of the treatment have remained challenging. This work aimed to produce and propagate an atmospheric pressure helium plasma directly into a closed microfluidic device made of glass for in situ treatment. Results obtained through contact angle measurements within the microchannels demonstrated a uniform wettability treatment with increased hydrophilic properties after only 1 minute of plasma treatment. We studied the stability of the plasma treatment by storing the devices either in water or air and measuring the evolution of the contact angle with time. With storage in air, we achieved at least 1 day of stable contact angle at around 23°, while water storage prolonged this stability for up to 3 days. Storage in air resulted in a full recovery of the initial wettability state after 13 days. When devices are stored in water a partial loss of wettability is observed, leading to a new wetting condition at a contact angle of around 30° that is stable up to a remarkable 70 days. Contact angle results are further supported with X-ray photoelectron spectroscopy surface analysis which revealed that the two effective mechanisms for wettability alteration are cleaning and surface functionalization. This in-house setup enables the processing of already bonded microfluidic devices providing treatment of all the inner microchannel walls. In addition, we are able to direct plasma propagation with electrode positioning, thus providing a way for selective wettability treatment of complex geometries.
| Country | France |
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