# Wettability alteration of closed glass microfluidic devices by in situ plasma

Viktor Gredicak, Claire Douat, Aneta Slodczyk, Sébastien Dozias, Sophie

Roman

► To cite this version:

Viktor Gredicak, Claire Douat, Aneta Slodczyk, Sébastien Dozias, Sophie Roman. Wettability alteration of closed glass microfluidic devices by in situ plasma. Microfluidics and Nanofluidics, 2025, 29 (4), pp.22. 10.1007/s10404-025-02793-9. hal-05005021

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Submitted on 26 Mar 2025

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# Wettability alteration of closed glass microfluidic devices by in situ plasma

Viktor Gredicak<fnref n="1" />,<fnref n="2" />,<fnref n="3" /> · Claire Douat<fnref n="2" /> · Aneta Slodczyk<fnref n="1" />,<fnref n="3" /> · Sébastien Dozias<fnref n="2" /> · Sophie Roman<fnref n="1" />

Received: 4 December 2024 / Accepted: 17 February 2025

© The Author(s) 2025

## Abstract

Experimental research on microfluidic devices requires adequate control over surface parameters like wettability. Plasma has already been proven to be a promising tool for the control and alteration of the wettability of solid surfaces, yet its propagation in microfluidic devices and treatment stability remains challenging. Our idea is to produce and propagate an atmospheric pressure helium plasma directly into closed micrometer-size glass channels for in situ wettability treatment. This approach enables better control over the treatment parameters compared to conventional treatments in low-pressure chamber-type plasma reactors. With a homemade kHz dielectric barrier discharge-like setup, we successfully propagated plasma through a 4 cm long rectangular microchannel of uniform depth (100 µm) and variable width (250–500 µm). Results obtained by in situ contact angle measurement on images indicate uniform wettability treatment with increased hydrophilic properties after only 1 min of treatment. The wettability achieved on a glass with our setup offers stability for up to 70 days depending on the plasma treatment and storage parameters. Contact angle results are further supported with X-ray photo-electron spectroscopy (XPS) surface analysis which revealed that the two effective mechanisms for wettability alteration are cleaning and surface functionalization.

**Keywords** Glass · Wettability alteration · Helium plasma · In situ plasma

## 1 Introduction

Microfluidics is a well-known research technique used to recreate microporous environments suitable for biomedical research (Maria et al. 2017; Sackmann et al. 2014), chemical

synthesis (Elvira et al. 2013; Kakuta et al. 2001), geo-environmental research (Chalbaud et al. 2007; Hu et al. 2017), water treatment (Patinglag et al. 2019) and so much more (Kazan 2022; Harshey et al. 2023). The functionalities of microfluidic devices are highly dependent on the bulk material, geometrical features, and surface characteristics (Kazan 2022). Controlling surface parameters like wettability is of particular interest. Wettability is often regarded as the ability of one fluid to spread on or adhere to a solid surface compared to another fluid. It is a product of both physical (e.g., surface roughness) and chemical surface features (e.g., surface functional groups), coming from the material and manufacturing process. Wettability is usually expressed through the concept of contact angle (θ), the angle between a fluid with respect to a solid surface and surrounding fluid. Strongly wetting conditions for a certain fluid are typically θ close to 0°, while weak or neutral wettability is considered for contact angles around 90°. Depending on the application requirements, the wettability characteristics of microfluidic devices may require alteration. One frequent reason for tuning the wettability is the difference between the natural

<figure><img src="image_2.png" /></figure>

Sophie Roman
sophie.roman@univ-orleans.fr

Viktor Gredicak
viktor.gredicak@cnrs-orleans.fr

Claire Douat
claire.douat@univ-orleans.fr

Aneta Slodczyk
aneta.slodczyk@cnrs-orleans.fr

Sébastien Dozias
sebastien.dozias@univ-orleans.fr

<footnote n="1">1 ISTO, UMR 7327, Univ Orléans, CNRS, BRGM, OSUC, 45071 Orléans, France</footnote>

<footnote n="2">2 GREMI, UMR 7344, Univ. Orléans, CNRS, 45067 Orléans, France</footnote>

<footnote n="3">3 CEMHTI, UPR 3079, CNRS, 45071 Orléans, France</footnote>
process environment and (synthetic) materials used to pro-
duce microfluidic devices.

An instance highlighting the significance of wettability pertains to the injection process of CO₂ for carbon capture and storage (CCS) objectives. In this particular case, the wettability takes part in governing the fluid flow behavior and the distribution of fluids in a multiphase system, and it can affect mass transfer and reaction rates (Agaoglu et al. 2015). Moreover, in biomedical research, wettability can affect the deposition of cells or different biomolecules (Sun et al. 2021; Hartatiek et al. 2024). Thus, it is important to adapt the wettability of microfluidic devices according to specific research applications.

To tailor the wettability of microfluidic devices there are several options, however it is important to note that discrepancies in reported values of achieved contact angles can be substantial (Iglauer et al. 2014; Gu et al. 2010; Bryk et al. 2020). As a first option, one can choose the material whose wettability characteristics are already fulfilling research requirements. There are several materials to choose such as silicon, glass, and various polymers. Glass (Iglauer et al. 2014) is expected to portray strong to moderate water-wet behavior, PMMA (polymethyl methacrylate) (Anbari et al. 2018) weak water-wet characteristics, while untreated PDMS (poly-dimethyl-siloxane) (Anbari et al. 2018; Tranidou et al. 2017) and certain fluorinated acrylate resins (Mannel et al. 2021) exhibit non-water-wetting features. Since changing or selecting a different material is not always suitable, there are several wettability alteration techniques available, e.g. chemical coatings, plasma, and UV treatments, or a combination of the mentioned techniques (Raj and Chakraborty 2020).

One of the most common chemical deposition processes in microfluidics is surface functionalization with silane self-assembling monolayers (SAMs) (Ansari and Imoukhuede 2018; Raj and Chakraborty 2020). Silanization, like other chemical deposition processes, offers good stability and plenty of chemicals to achieve diversity in wettability states from hydrophilic to hydrophobic (Arkles 2011; Glass et al. 2011). However, there are some concerns associated with this group of methods. These typically include long procedure duration, elevated temperature requirements, low environmental friendliness and work safety, chemical cost, variable deposition thickness, low flexibility in achievable contact angle, and possible repeatability issues (due to concentration and/or aging of the chemicals) (Ward et al. 2005; Glass et al. 2011; Goncalves Canane 2019; Holmberg and Matthews 2009; Musbah et al. 2022). Thus, chemical treatments are often plasma-assisted or UV-aided to provide better control over the wettability and speed up the deposition process (Vandenbossche and Hegemann 2018). Still, merging the two techniques may add complexity to the process

and financially strain the project due to the required equip-
ment and consumables.

Wettability alteration can also be achieved using plasma technology without additional chemicals, simplifying the process. A convenient way to ignite plasma for microfluidics is to apply high voltage in a gas (noble gases with or without addition of molecular gases) (Terpilowski and Rymuszka 2016). Due to the high-energy nature of the gas in the plasma state, plasma itself (free electrons and ions) can prompt surface changes. The mechanisms of these changes are attributed to chemical and physical decontamination, modification of surface topology and/or surface functionalization (Lin et al. 2021; Turkoglu Sasmazel et al. 2021). For microfluidic applications, non-equilibrium low-temperature atmospheric and low-pressure plasmas are well suited. By definition, non-equilibrium plasma features different temperatures for each species. The ion and neutral species temperatures are nearly identical, typically reaching a few hundred Kelvin, while the electron temperature is significantly higher, often rising to several electronvolts, equivalent to tens of thousands of Kelvin. This characteristic presents a notable advantage: the high-energy electrons drive rich chemical reactions and induce wettability alteration, while the ions and neutral species maintain temperatures close to room temperature, preserving the bulk material's properties. Thus, the treatment time with plasma is typically in the order of tens of seconds (Li et al. 2020; Lin et al. 2021; Wagner et al. 2003) which is substantially faster compared to chemical deposition procedures that can take hours to complete (Mansouri-Boroujeni 2022). The achievable change of contact angle depends on the material and treatment process itself, with some studies demonstrating even a 100° of reduction (Dou et al. 2021; Morent et al. 2010; Tan et al. 2010). However, a great challenge with all surface modification techniques, but especially pure plasma treatment, is wettability aging (Kovach et al. 2014; Massey et al. 2012; Trantidou et al. 2017).

Wettability aging, or hydrophobic recovery, is a phenomenon where newly created wettability conditions deteriorate over time, usually towards the hydrophobic end of the spectrum. Depending on the material, the type of wettability treatment, and experimental/storage parameters, a number of mechanisms for wettability deterioration have been proposed (Princ and Mozetic 2022). Effectively, factors like experimental and storage fluids, humidity, temperature, and even light affect the stability of certain functional groups which play a big role in controlling the wettability (Dou et al. 2021; Vandenbossche and Hegemann 2018; Jahangiri et al. 2020). Thus, the time frame of stability may be vastly different ranging from a few hours (Kim et al. 2015; Mansouri-Boroujeni 2022; Tan et al. 2010) to days (Princ and Mozetic 2022; Vandenbossche and Hegemann 2018; Massey et al. 2012). On the whole, regardless of the way it is

<figure><img src="image_3.png" /></figure>
achieved, having stable wettability conditions is an absolute necessity for representative and repeatable experiments.

Applications of plasma treatments for wettability alteration of microfluidics devices have been limited to open microfluidics (Wu et al. 2019a) and treatments before bonding (Bhattacharya et al. 2005), predominantly on polymer surfaces (Subedi et al. 2024) like PDMS (Li et al. 2024) which have pronounced wettability aging effects leading to stability in the order of couple of hours (Mansouri-Boroujeni 2022; Tan et al. 2010). These treatments are commonly performed in low-pressure oven/chamber type reactors associated with low control over the treatment process. To elaborate, the charged particles of plasma (electrons and ions) are not free from each other. Their motion depends on the electric field produced by them. They are governed by a collective behavior. Debye length is the characteristic length of the electric-field screening, in other words, a charge can affect the other charges located in a sphere with a radius of the Debye length. At low pressure, the Debye length is commonly higher than 100 µm (Lieberman and Lichtenberg 2005), which is the typical dimension of microchannels. A collective behavior of the charges is necessary to get a plasma, and this collective behavior is only possible when the Debye length is much lower than the dimension of confining space. Consequently, low-pressure plasma is not appropriate for the treatment of closed microfluidic devices, requiring the use of plasmas with much lower Debye length. The latter can be reduced when the pressure of the gas increases. For example, the typical Debye length of a helium non-equilibrium atmospheric pressure plasma is in the range of a couple of tens µm (Wu et al. 2023) and could be used in closed microchannels.

A plasma jet is a non-equilibrium plasma at atmospheric pressure that can propagate a plasma outside the electrode confinement. They can be used for local applications, providing them with better control over the treatment parameters (Lin et al. 2022; Szili et al. 2011; Yamasaki et al. 2013). They can be produced in a variety of configurations of setup, allowing them flexibility to be coupled with microfluidics (Lin et al. 2022; Priest et al. 2011). Some experimental and numerical studies have been performed on capillaries of various sizes to support the claim that plasma can propagate through narrow channels. As shown in the Table 1, propagation is achievable in tubes with millimeter-scale dimensions, extending down to capillaries just a few hundred nanometers in size. However, none of these studies examined the impact of such plasmas on the wettability changes. To fill this gap, we designed plasma sources based on the plasma jet configuration to treat microfluidic devices in situ to study the alteration of wettability.

To the best of our knowledge, in situ wettability alteration using plasma-jet technology has never been performed on closed glass microfluidic devices. Thus, in this paper, we propose a new practical method to alter the wettability of such microfluidic devices by using plain helium plasma jet technology. Our aim is to assess the feasibility of propagating plasma at atmospheric pressure in glass microfluidic systems with rectangular channels (down to 250 × 100 µm) and evaluate its effects on wettability alteration over time.

<table><caption>**Table 1** Review of plasma propagation capabilities in confined geometries</caption><thead><tr><td>Author</td><td>Approach and plasma gas</td><td>Confinement</td><td>Propagation length</td></tr></thead><tbody><tr><td>Johnson et al. (2011)</td><td>Exp. He</td><td>1.6–9.5 mm PTFE tube</td><td>@ ID 4.8 mm → 20 cm</td></tr><tr><td>Robert et al. (2013)</td><td>Exp. Ne</td><td>4 mm glass capillary</td><td>1 m</td></tr><tr><td>Xu et al. (2015)</td><td>Exp. He/Ar + O<sub>2</sub></td><td>4 mm PVC tube</td><td>Not mentioned</td></tr><tr><td>Xiong et al. (2012)</td><td>Exp. and Num. Ne</td><td>4 mm glass capillary</td><td>20 cm</td></tr><tr><td>Onyshchenko et al. (2015)</td><td>Exp. Ar</td><td>0.28–3 mm polyethylene tubes</td><td>@ ID 0.28 mm → 10 cm</td></tr><tr><td>Wang et al. (2020)</td><td>Exp. He + O<sub>2</sub></td><td>2 mm PTFE tube</td><td>30 cm</td></tr><tr><td>Brahme et al. (2018)</td><td>Exp. He and Ar</td><td>100 µm–1.5 mm Pyrex capillaries</td><td>minimum ID 250 µm (He)/150 µm (Ar)</td></tr><tr><td>Wu et al. (2016)</td><td>Exp. He</td><td>0.07–4 mm quartz tubes</td><td>@ ID 0.6 mm → 6 cm</td></tr><tr><td>Jain and Bruggeman (2023)</td><td>Exp. He</td><td>75–500 µm Pyrex glass capillaries</td><td>Not mentioned</td></tr><tr><td>Wu et al. (2023)</td><td>Num. He</td><td>0.05–1 mm quartz tubes</td><td>Not mentioned</td></tr><tr><td>Wu et al. (2019b)</td><td>Exp. Ar</td><td>4–100 µm quartz tubes</td><td>@ ID 100 µm → 59 mm</td></tr><tr><td>Wu et al. (2018)</td><td>Exp. Ar</td><td>10 µm quartz tube</td><td>@ 23.6 kV → 1.5 mm</td></tr><tr><td>Kakei et al. (2010)</td><td>Exp. He</td><td>100 nm–5 µm glass capillaries</td><td>Not mentioned</td></tr></tbody></table>

All reviewed plasmas are atmospheric pressure plasma jets sorted with increasing confinement. Exp., Num., PTFE, PVC, and ID stand for experimental, numerical, teflon, polyvinyl chloride, and inner diameter, respectively
# 2 Materials and methods

In the following section, equipment, materials, and experimental procedures are introduced. The equipment for this study consists of two platforms, microfluidic and plasma platforms, further described in the following text.

## 2.1 Microfluidic platform

The principal component of the platform is borosilicate glass microfluidic device manufactured by Micronit (The Netherlands). Glass has been chosen as it is a widely used material in microfluidics, it has good mechanical, optical, and stable hydrophilic features compared to PDMS. The microfluidic device geometry is made of a square pocket connected to a straight microchannel, see Fig. 1, that is representative of dead-end pores in porous media, e.g. (Roman et al. 2020). Three 4 cm long microchannels are etched in each microfluidic device with a uniform depth of 100 µm. The width depends on the location: 500 µm in the large channel section, 250 µm in the small channel section, while the side of square pocket equals 500 µm (see Fig. 1).

For contact angle determination, the microfluidic device is placed under a microscope (Nikon Eclipse Ni-U upright microscope) equipped with a motorized stage (Prior Scientific PS3J100). The light source is a mercury lamp (Nikon Intensilight C-HGFI), and a camera (Andor Neo, sCMOS 5.50 Megapixel) is used to acquire high-resolution images. All images are acquired with objective 20x with a numerical aperture of 0.45. An in situ static contact angle between water (MilliQ - 18.2 Ωcm) and CO₂ (99.995 % purity) is

<figure><img src="image_4.png" /><figcaption>**Fig. 1** Image of glass microfluidic device during plasma treatment and the simplified schematics with the dimensions of microchannels. The entire geometry has a rectangular cross-section with a uniform depth of 100 µm. There are three microchannels etched in each microfluidic device. For visual clarity, only one is shown</figcaption></figure>

measured on acquired images with the help of ImageJ plugin called DropSnake (Stalder et al. 2006).

## 2.2 Plasma platform

The plasma source is a dielectric barrier discharge-like jet, depicted in Fig. 2. The powered electrode (anode) is a stainless steel needle (gauge 20 from Nordson) attached to the microchannel via custom 3D printed plastic clamp. Close to the outlet of a microchannel, a grounded copper foil electrode is glued (cathode) to enhance the plasma propagation. The device is powered with positive microsecond-duration voltage pulses from 10 to 15 kV at 8 kHz, produced by a homemade power supply similar to the one used by Douat et al. (2021). An example of the high voltage waveform is given in the supplementary information (Fig. 14). Helium (99.999 % purity) gas is flown through the high voltage electrode at 50 standard cubic centimeter per minute (sccm) regulated by a calibrated mass flow controller (EL-FLOW Prestige from Bronkhorst). The type of plasma created is characterized as helium non-thermal atmospheric pressure plasma jet. A fast CCD camera (pco.pixelfly usb) with objective 10x and NA of 0.28 (Edmund optics) is deployed for plasma imaging. Additionally, light from the plasma is collected via an optical fiber and analyzed by an optical emission spectrometer (AvaSpecULS4096CL) to acquire information about created plasma species. To measure the temperature on the surface of glass microfluidic device, optic fiber temperature sensor (GaAs Optocon) is deployed. Only when measuring the power, a resistance of 1 kΩ is added to the grounded electrode. The applied voltage and the voltage through the resistance were, respectively, measured using a Tektronix P6015 probe (75 MHz, and 1000:1 division ratio) and a Elditest GE3121 probe (150 MHz and 100:1 division ratio). The signals were recorded using a 500 MHz bandwidth digital oscilloscope (LeCroy WaveRunner 6050).

<figure><img src="image_5.png" /><figcaption>**Fig. 2** Atmospheric pressure plasma jet setup composed of gas supply part, high voltage unit and measuring instruments</figcaption></figure>

<figure><img src="image_6.png" /></figure>
## 2.3 X-ray photoelectron spectrometry (XPS)

To characterize chemical changes made by the plasma, X-ray photoelectron spectrometry (XPS) analysis is deployed. Since glass microfluidic devices are tightly bonded, equivalent plasma treatment is performed on the top glass surface with slightly different setup configuration (see Supplementary information Fig. 10). XPS measurements are performed using an ESCALAB Xi+ X-ray photoelectron spectrometer (Thermo Fisher Scientific) employing a monochromated Al Kα X-ray source (hv = 1486.6 eV). The carbon C(1s) level (284.8 eV) is used as the reference binding energy. Both general survey and high-resolution spectra are collected using an analysis area of 650 × 650 µm and a 20 eV pass energy. The charge neutralizer is used for data collection, being monitored using the C(1s) signal corresponding to adventitious carbon (Biesinger 2021). All spectra are collected and fitted using the Avantage software (ThermoScientific).

## 2.4 Experimental procedure

The experimental procedure is composed of 3 major steps: (1) plasma propagation through the microchannel to alter the wettability of walls; (2) microfluidic experiments to acquire an image of an interface between CO₂ gas and water, and (3) image preprocessing and contact angle measurement, described as follows. The entire procedure is performed in laboratory with the air conditioning unit set to 20 °C.

### Step 1 - Plasma treatment

To reduce the impurity level from the tubing outgassing and to work in reproducible conditions, before any plasma treatment, helium is set to flow for at least 30 min at 50 sccm. Afterwards, the plasma is ignited and the plasma propagation length increases with the magnitude of the applied voltage. The high voltage is set when the whole microchannel is filled with plasma. Typically, it requires a voltage from 10 to 15 kV. As the outlet of the microchannel is open to the atmosphere, the plasma continues its propagation in air along the helium channel and forms what we call a plasma plume, Fig. 2. When the plasma is on, images of plasma propagation, optical emission spectra (OES), plasma power and temperature readings are acquired. Once the treatment is done (from 30 s to 10 min), plasma is turned off and the microfluidic device is stored in water or in air. Microfluidic devices intended for gas (air) storage are left in a small plastic box, while devices intended for water storage are first saturated with water and then stored in a container full of MilliQ water. The day of plasma treatment is denoted as day 0.

### Step 2 - Microfluidic experiments and image acquisition

The wettability of microchannels is checked before and periodically after plasma treatment. Under a camera and a microscope, the testing procedure is performed as follows, see Fig. 3. Using a ⊥-junction to ensure a closed system at the inlet, water and CO₂ are injected into the tubing one at a time to form a small volume of CO₂ sandwiched between two larger slugs of water. By careful manual injection of water, the water-CO₂ interface is brought from tubing to the inlet side of the 500 µm channel where images are taken. When the interface between water and CO₂ is quasi static within the camera window, image acquisitions are made. It is more difficult to stabilize the interface when the contact angle is low, due to the existence of lubricating water films over which the interface is easily sliding.

### Step 3 - Contact angle measurements

Acquired images are pre-processed with ImageJ software. Images are first converted to 8-bit gray scale format and adjustments of brightness and contrast are performed to enhance the visibility of the interface and contact points, Fig. 4. For the contact angle determination, Drop-Snake plugin is utilized. DropSnake was developed by Stalder et al. (2006) and is based on the snake algorithm. The principle of DropSnake plugin is to set the dots fitting the interface which are then interpolated by the algorithm. On average 12 points have been placed to describe the interface which is above recommended 10 stated by the creators of the DropSnake tool. Two contact angle readings ($\theta_1$ and $\theta_2$) are obtained for each image, see Fig. 4. For statistical significance, no less than 5 images are utilized to calculate each data point of contact angle. The dispersion of measured results is expressed through standard deviation (error bars graphically). It is attributed to advanced and receded state of the interface (contact angle hysteresis) (Marmur et al. 2017; Huh and Scriven 1971; Hansen et al. 2000), manual input of the dots affected by interface and

<figure><img src="image_7.png" /><figcaption>**Fig. 3** Fluid injection sequence for in situ contact angle measurement. A bubble of CO₂ is injected through a ⊥-valve and brought to microfluidic device's large channel within the camera window for image acquisition</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4** Measurement of contact angle with DropSnake plugin. Image a) depicts CO₂-water interface before plasma treatment and image b) after with their respective contact angle readings ($\theta_1$ and $\theta_2$)</figcaption></figure>

contact point visibility, and accuracy of the tool itself. Overall, readings typically oscillate ± 5°, in agreement with the accuracy of contact angle measurements in microfluidic devices (Rooijen et al. 2022).

## 3 Results and discussion

We analyze several aspects of the wettability alteration by atmospheric pressure plasma which are further elaborated in the following sections. At first, we evaluate plasma propagation visually to get a sense of treatment uniformity. Simultaneously, optical emission spectroscopy is deployed to survey the composition of plasma species and thus anticipate the possible changes in wettability. At the core of this paper, we are evaluating the wettability performance of plasma treatment through contact angle, as a function of treatment time, aging time, and storage media. Finally, XPS is utilized to determine the effect of plasma on surface chemical composition.

### 3.1 Plasma properties in microfluidic devices

By optimization of the voltage to avoid arcing between the two electrodes, plasma is successfully ignited and then propagated through the entire length of the microchannel (4 cm). Unlike many previous studies where the propagation is followed in rather simple tube geometry, see Table 1, our microchannel features a pocket structure and a varying rectangular cross-section, introducing greater geometric complexity and making plasma propagation more challenging. With stable propagation of plasma from the inlet to the outlet, close-up images are acquired to provide better insight into the propagation at the micro-scale. In addition to the main setup configuration with the power electrode at the inlet, Fig. 2, another configuration with the needle power electrode placed at the outlet is prepared to unveil the impact of electrode position on plasma propagation. The latter configuration does not include the ground electrode on the opposite side of the microfluidic device as it is not possible to configure it in the setup.

**Light emitted by plasma distribution.** As visible from stitched images (Preibisch et al. 2009) in Fig. 5, the light emitted by plasma is generally brighter in the middle of the channel. This is contrary to the literature (Mericam-Bourdet et al. 2009; Douat et al. 2016; Leiweke et al. 2011), where light distribution in plasma plume has a ring shape (more intense light emission on the edges). The ring shape has been explained by numerical models (Breden et al. 2012; Boeuf et al. 2013) which attribute the annular structure to a charge accumulation on the dielectric surface forming a surface discharge. Wu et al. (2023) clarified that central column propagation of plasma, similar to our case, disappears to become a ring structure when the width of the streamer head, which is on the order of a few 100 µm, is smaller than the diameter of the channel (Xiong and Kushner 2012). In our case, the channel has a rectangle shape with a dimension of 100 × 500 µm and we do not observe a surface discharge, because the width of the channel is lower than the range of streamer width.

Furthermore, we observed decreasing light intensity as a function of distance from the power electrode and increasing as a function of confinement. In their 2D numerical model, Wu et al. (2023) recently demonstrated the decrease of electric field and electron density due to continuous charge losses to the side wall and increase with reducing the tube diameter, supporting our observations.

Between the two electrode configurations, the light intensity is higher in the case represented in Fig. 5b than in Fig. 5c. The reason is that in case of configuration in Fig. 5c there is no grounded electrode located on the other side of the micro-channel. With no grounded electrode,
<figure><img src="image_9.png" /><figcaption>**Fig. 5** Plasma propagation and distribution across the microfluidic device under 12 kV of applied voltage. Each image is accumulated over several plasma cycles (pulses). Images **b), c)** are accumulated over 100 ms period and indicate two different electrode configurations as indicated. These two images are stitched from single images as denoted on schema (**a**). Images **d) and e)** are single images accumulated over 250 ms period to better indicate plasma propagation behavior in the pocket for representative configurations **b), c)** respectively</figcaption></figure>

the electric field is not enhanced which directly impacts
the light intensity.

**Direction of the flow.** When looking at the impact of gas flow relative to the electrode positioning, we observe that regardless of the flow direction plasma propagates from the powered electrode towards the opposite side of the channel. Considering that the characteristic plasma propagation time is in the range of hundreds of ns (Xiong and Kushner 2012) and the characteristic gas flow time through the length of the microfluidic device in the ms range, the gas can be assumed to be static during plasma propagation, which explains why plasma propagation does not depend on the gas flow direction.

**Plasma propagation in the pocket.** In the pocket, the light is weaker compared to the underlying channel and it forms a triangle shape (see Fig. 5d) with a side stuck near the wall opposite to the position of the powered electrode. As we already disclosed, the plasma does not depend on the gas flow direction. It has been shown that the electron distribution of the ionization wave hugs the wall, generally on the side of the wall with a smaller radius of curvature, and depends on the local instantaneous electric field (Xiong and Kushner 2012). In our case, it suggests that the electric field parallel to the axis of the channel is stronger than the perpendicular electric field.
<figure><img src="image_10.png" /><figcaption>**Fig. 6** Optical emission spectrum acquired at the inlet side of the large channel with the configuration visible on Fig. 5a under 14 kV. The spectrum is acquired with an acquisition time of 500 ms and normalized to the intensity value of helium line at 706.5 nm indicated in red on the graph. "C-B", "B-A" and "B-X" notation represent the de-excitation process in specific molecular energy states</figcaption></figure>

**Optical Emission spectroscopy.** The in situ spectral analysis of plasma species by OES in Fig. 6 indicates a number of significantly intense lines. Lines at 587.6 and 706.5 nm are attributed to helium which is expected as the main gas in the system. Thus, for ease of reference, the whole spectrum is normalized to the intensity of helium line at 706.5 nm. Besides helium, the spectrum is also characterized by the strong intensity of the second positive system of nitrogen, N₂ (C-B), from 315 to 486 nm (Pearse and Gaydon 1976). Additionally, within this range, strong presence of N₂⁺ (B-X) species is indicated with a presence of line at 391.4 nm, while cyano radical (CN (B-A)) (Pearse and Gaydon 1976; Nemes et al. 2000) is identified by the presence of a band with heads located at 386.1 and 388.3 nm. In either the position of the needle power electrode at the inlet (Fig. 5b) or at the outlet (Fig. 5c), CN band is observed for an applied voltage higher than 10 kV, which suggests that energetic electrons are required to produce this molecule. If the microfluidic device is removed, the needle power electrode produces a thin plasma jet propagating in air. The emission spectrum of this plasma jet reveals no CN band, which indicates that CN formation is not due to the presence of N₂ and CO₂ in air. As CN band was only observed with the microfluidic device, we concluded that carbon must come from the interaction of plasma with the borosilicate glass. More about CN band identification is shown in supplementary materials. The presence of N₂⁺ (B-X) and CN (B-A) species is a good indication of Penning ionization in the system with metastable helium atoms and possible plasma-enhanced chemical reactions, respectively. The remaining strong lines are attributed to hydrogen at 656.3 nm (Hα) and oxygen triplet from 777.2

to 777.5 nm. The presence of nitrogen, oxygen, and hydrogen is attributed to the trace amount of impurities coming from the air due to tubing outgassing, possible leaks, and bottle impurities. The intensity of lines and bands decreases with the distance and increases with confinement which is consistent with the previous findings by the plasma imaging shown in Fig. 5.

**Plasma power and temperature measurements.** We ran power and temperature measurements to estimate the impact of plasma on the material. A detailed description of both can be found in the supplementary information. At 12 kV (representative of typical plasma treatment), energy delivered to the plasma in single electric pulse is between 60 and 70 µJ. Given the frequency of 8 kHz, the power produced by plasma is in order of 500 mW. Under these plasma conditions, the temperature of the outer glass surface was measured near the inlet, where the powered electrode is located, and near the outlet, close to the grounded electrode. After 5 min of treatment, the glass was hotter near the inlet, reaching 55 °C, while the temperature near the outlet was around 40 °C. Given the thermal stability of borosilicate glass, which undergoes transition behavior only above 500 °C, no negative thermal effects on the mechanical properties of glass microfluidic devices are expected. Since the material temperatures remain well below 100 °C, this finding extends to polymers, which are more sensitive to thermal loads. Considering the relatively low treatment temperature and short duration, our treatment approach is expected to be compatible with polymeric microfluidic materials such as PDMS.
The average gas temperature in the plasma region was estimated using the thermal Ohms law, which relates the temperature difference across a material, in this case, the borosilicate glass plate, to heat transfer. Since most of the energy delivered to the plasma is converted into thermal energy, we can assume, as a first approximation, that all the energy is transformed into heat. Consequently, the heat transfer is equal to the power measured from electrical waveform signals. Temperature range of plasma gas is thus between 60 and 70 °C, which is in accordance with the literature on atmospheric pressure plasma jet in helium (Douat et al. 2016; Chang et al. 2012; Jogi et al. 2014).

In conclusion, our helium atmospheric pressure plasma is powerful for surface modification of glass microfluidic devices, yet non-destructive to the bulk material. With the guided propagation, the plasma treatment is expected to give uniform wettability results in the microchannel. Based on literature and OES results, surface cleaning and surface functionalization are expected to be the main drivers of wettability alteration. Further analyses will be given in subsection "Discussion on wettability alteration and aging mechanisms".

## 3.2 Effect of plasma treatment time on wettability alteration

After the successful plasma propagation, our next goal is to determine the effect of plasma on the surface wettability. Several channels are treated with different plasma exposure times and then analyzed for the contact angle soon after the plasma treatment (less than 2 h of delay). From Fig. 7 it is visible that the contact angle drops from 53 ± 7° to 23 ± 5° in the first 60 s of treatment giving on average a 30° reduction of contact angle. Regardless of increased exposure time (up

<figure><img src="image_11.png" /><figcaption>**Fig. 7** Contact angle evolution with plasma treatment time with hand-drawn eye-guide lines indicating the behavior</figcaption></figure>

to 600 s), contact angle stays stable indicating that maximum potential of the treatment has been reached with just 60 s of treatment. Based on these measurements, we choose an optimized treatment time of 120 s minimum and decide to further examine the effect of treatment time on contact angle stability over storage time, described in the next section.

## 3.3 Aging of plasma-altered wettability

Knowing the fact that wettability treatment is not permanent (Mansouri-Boroujeni 2022; Tan et al. 2010; Primc and Mozetic 2022), we decided to track the contact angle time evolution when the microfluidic device is stored in air and water. As seen in Fig. 8, the wettability is stable for a minimum of 1 day in the air and at least 3 days in water, ensuring representative average wettability conditions of 23 ± 5° in this period. The contact angles measured on both the inlet and outlet sides of the microchannel have the same values within the error margin confirming the uniform treatment along the channel. More is shown in supplementary information under subsection 3 - Wettability uniformity. Considering that the effects of plasma wettability treatments usually last for only a couple of hours and that microfluidic experiments frequently last 1 day or less (Mansouri-Boroujeni 2022; Tan et al. 2010; Primc and Mozetic 2022), both water and air are suitable as experimental and storage fluids after our plasma treatment. Thus, atmospheric in situ plasma treatment is a new technique to increase the hydrophilic properties of glass microfluidic devices.

In the case of air storage (Fig. 8a and 8b), wettability started deteriorating after the first day reaching before-plasma conditions around day 13. For the remaining duration of testing, wettability stayed stable as expected. The same behavior is observed if a microfluidic device is re-treated after 23 days of air storage, see Fig. 8b. Stating the high price and difficulty of producing glass microfluidic devices, the possibility to retreat and reuse the device is certainly advantageous.

Aging in water (Fig. 8c) is characterized by a stable water-wetness for the first 3 days after the plasma treatment ($\theta = 23 \pm 5°$). Then, the contact angle rises to an average of 31 ± 8° and remains stable for up to 70 days post-treatment. This is a valuable finding considering that water is one of the most frequently used fluids for microfluidic experiments.

Overall, increasing treatment time up to 10 min had no evident effects on the aging performance in any of the two tested storage media, Fig. 8. Moreover, our aging results in glass microfluidic devices follow similar general trends observed on other flat and open surfaces by Dou et al. (2021) and Vizireanu et al. (2017). These results demonstrate the good stability of our treatment and highlight the need for further investigation into the mechanisms underlying the alteration.
<figure><img src="image_12.png" /><figcaption>**Fig. 8** Aging of wettability in air (a, b) and water storage (c) after 2, 5 or 10 min of plasma treatment. Results displayed in graphs a), c) are obtained on first-time-used microfluidic devices while aging in case b) is analyzed on re-used devices. Lines within the data represent eye-guide lines drawn by hand and not the fitted trendlines. The data point at day 0 corresponds to non-treated microfluidic devices</figcaption></figure>

### 3.4 Discussion on wettability alteration and aging mechanisms

To unravel the mechanisms of wettability alteration and aging, we conducted chemical analysis on the microfluidic device glass surfaces with the help of XPS. In addition to the non-treated reference sample, we tested 4 other conditions. To showcase the effect of plasma on surface wettability without the signs of aging (day 0) and outline possible differences in the applied voltage during the plasma treatment (10–15 kV) we prepared and tested two samples accordingly. The other two samples are prepared to reveal the different aging mechanisms in air and water. To compare between the samples, we showcase our results as (1) processed XPS general survey and high-resolution spectra visible in Fig. 9, and (2) elemental composition comparison of normalized area under the curve for three selected elements (oxygen, carbon, and nitrogen) depicted in Table 2.

**Mechanisms of wettability alteration.** In the general survey spectrum of reference (non-treated) sample (Fig. 9a), the presence of some principal and minor glass elements is confirmed. Some of these include: silicon (103 eV), oxygen (532 eV), carbon (285 eV), boron (192 eV), zinc (1022 and 1045 eV), aluminum (74 eV), sodium (1073 eV) and nitrogen (399 eV). Note that hydrogen and helium are the only elements not directly detectable by the XPS analysis. By comparing the reference general survey spectrum to any plasma-treated sample (Fig. 9a), we observe predominantly quantitative changes in the elemental composition of the glass surface. Since oxygen, carbon and nitrogen are previously determined by the OES (see Fig. 6) to be a part of plasma and are thus assumed to be primary drivers of the wettability alteration (Vandenbossche and Hegemann 2018), hence, further analysis with high-resolution spectra is performed on these three elements. Analyzed in that manner, the surface of non-treated samples is composed primarily of oxygen (84.2 %), some carbon (14.9 %), and a small amount of nitrogen (0.9 %).

By looking at Table 2, the first global observation from the XPS analysis indicates an increase of oxygen and a decrease of carbon and nitrogen content on the surfaces of all plasma-treated samples regardless of treatment or aging parameters. By the decomposition of the high-resolution spectra, we confirmed that oxygen is largely present in the bulk structure of the glass (Si–O and B–O bonds) while a substantial amount of carbon and nitrogen are present as removable surface contamination, see Fig. 9b, c. Thus, we attribute this observation to the plasma cleaning effect which exposes the fresh uncontaminated glass surface full of oxygen atoms while physically and chemically removing the contaminants (carbon and nitrogen compounds). Consequently, the exposure of oxygen groups renders the surface
<figure><img src="image_13.png" /><figcaption>**Table 2** Elemental composition comparison of the glass surface for 5 tested samples with corresponding treatment parameters and wettability values. Indicated elemental values are taken as the area under the curve from high-resolution XPS spectra and further normalized on the reference spectrum for each element. Note that the area under the curve is proportional to the amount of elements on the surface</figcaption></figure>

<table><thead><tr><th>Sample</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th></tr></thead><tbody><tr><td>Plasma (Yes/No)</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>Voltage (kV)</td><td>/</td><td>10</td><td>15</td><td>15</td><td>15</td></tr><tr><td>Aging (Yes/No)</td><td>/</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td></tr><tr><td>Storage medium</td><td>/</td><td>/</td><td>/</td><td>Air</td><td>Water</td></tr><tr><td>Contact angle (°)</td><td>53</td><td>23</td><td>23</td><td>42</td><td>31</td></tr><tr><td>Carbon</td><td>1.00</td><td>0.52</td><td>0.53</td><td>0.59</td><td>0.61</td></tr><tr><td>Oxygen</td><td>1.00</td><td>1.39</td><td>1.37</td><td>1.32</td><td>1.28</td></tr><tr><td>Nitrogen</td><td>1.00</td><td>0.65</td><td>1.26</td><td>0.74</td><td>0.39</td></tr></tbody></table>

more hydrophilic. Since oxygen and carbon are the two dominant constituents of the glass surface in this reduced element analysis, the cleaning mechanism is assumed to be the primary driver of wettability alteration. Wettability treatment seems to be independent of applied voltage as the oxygen and carbon levels between 10 and 15 kV treated samples do not change substantially (Table 2).

Nitrogen levels across all plasma-treated samples tend to be lower than the reference (Table 2). As mentioned earlier, this is an indication of nitrogen being a part of the surface contaminants removed by the plasma. When comparing 10 and 15 kV samples treated on day 0, 15 kV sample has higher levels of nitrogen. This observation seems to be related and consistent with OES findings where we observe an increase of nitrile/cyanogen groups due to a reaction between ionized
nitrogen and carbon present on the surface. Overall, it is likely possible that nitrogen and carbon surface contamination is converted into surface functionalization under the effects of plasma. Hence, the analysis of deconvoluted high-resolution spectra (Fig. 9b and 9c) is performed on both carbon and nitrogen to see possible shifts in binding energy that would indicate the formation of new chemical bonds.

The analysis of carbon peak (Fig. 9b) indicates a small increase in area under the curve of O-C=O bonding at 288.9 eV immediately after the plasma treatment with a stronger effect visible for 15 kV treatment. Since this sub-band is associated with the formation of carboxyl groups (COOH) (Egghe et al. 2022), part of wettability alteration can also be attributed to this functionalization mechanism. Other sub-bands tend to show a significant decrease confirming their function as surface carbon contamination and thus will not be further analyzed.

The analysis of nitrogen peak (Fig. 9c) indicates both qualitative (shift in bonding energy) and quantitative alteration (intensity / area-under-the-curve changes) of spectra across the tested conditions. Going from lower binding energies, the sub-band around 398 eV is present in both reference and all plasma-treated spectra with acceptable shift variations in binding energy (up to 0.3 eV). Thus, the sub-band is associated with strong nitrogen bonding to silicon and/or boron atoms in the glass. The sub-band at 401.2 eV in the reference spectrum disappears from all plasma-treated samples while the sub-band at 403.9 eV is likely being shifted to the lower bonding energy levels and subjected to changes in the area under the curve. Besides the sub-band at 398 eV, the whole region between 392 and 405 eV is attributed to organic compounds related to nitrile and/or amino and/or similar groups (Jansen and Van Bekkum 1995; Egghe et al. 2022; Aziz et al. 2017). Due to visible shifts and changes in the sub-band area of this region, we propose a double effect of plasma on the glass surfaces. Firstly, a part of organic compounds are converted to new compounds performing surface functionalization. This claim is supported by broadening of the whole nitrogen band of plasma-treated samples compared to the reference spectrum. Secondly, all other compounds that are not transformed and attached are likely being etched from the glass surface reducing the surface contamination with possibly less hydrophilic molecules.

**Mechanismas of aging in air and water.** Continuing analysis on nitrogen spectra, Fig. 9c, the highest energy sub-band around 407 eV is detected only in plasma-treated samples except for water stored sample. Based on Thermo Fisher and similar XPS databases, the sub-band is attributed to the formation of nitrates. The absence of this sub-band in water storage is a consequence of nitrates solubilization in water.

Less aging effects in water storage compared to the air storage can thus be explained as a synergetic effect of several

mechanisms. As just mentioned, the ability of water to hydrolyze certain nitrogen compounds is the primary reason for partial wettability loss. However, having an ultra-pure MilliQ water covering the glass surfaces prevents reactions and the return of hydrophobic contaminants that come from the environment (air). Furthermore, hydroxyl groups, which are integral components of carboxyl groups, are also present in water but not in air. Thus, water exerts a preserving effect on them elucidating a sustained wettability following the initial rise in contact angle.

Altogether, these XPS findings are consistent with the behaviour found in our wettability measurements and support the initial decrease (plasma treatment) and subsequent increase of contact angle (aging) with time in both storage fluids.

## 4 Conclusions

In this paper, we demonstrated novel application of plasma jet for wettability alteration of closed glass microfluidic devices. With our in-house developed setup, we successfully propagated helium plasma jet with minor amount of air impurities in 4 cm long microchannels of variable width. The in situ wettability treatment thus enables processing of already bonded microfluidic devices bought from companies, which is frequently the case for glass. The post-bonding approach also provides treatment of all microchannel walls and ensures minimum wettability degradation which may happen during the bonding process. In addition, the ability to affect plasma propagation with electrode positioning provides a simple way for selective wettability treatment of more complex geometries. The result of just 1 min of plasma treatment is a significant increase in hydrophilic properties of microchannel surfaces visible in contact angle drop of approximately 30°. Overall, fast treatment and low operating costs make it an attractive alternative compared to other wettability alteration techniques. With storage in the air, we achieved at least 1 day of stable contact angle at around 23°, while water storage prolonged this stability period for up to 3 days. Whereas storage in air results in full recovery after 13 days of wetting conditions, water storage experienced only partial loss of wettability resulting in a new wettability condition stable up to a remarkable 70 days. Conducted XPS analysis attributed the change in wettability to the surface cleaning and functionalization of the glass surfaces. Finally, due to non-permanent wettability alteration and a possibility of re-treatment with the same results, plasma jet platform offers flexibility in wettability requirements and the possibility to reuse difficult-to-manufacture glass microfluidic devices.

As an outlook for the future, the developed methodology can now be applied to other microfluidic device materials, for

<figure><img src="image_14.png" /></figure>
example to PDMS which is commonly used in microfluidics experiments. We anticipate covering both hydrophilic, neutral, and hydrophobic parts of the wettability spectrum and extending the stability of newly created wettability conditions with the introduction of specific desired admixtures to the plasma gas. Furthermore, adapting the electrode position according to the geometry features of the microfluidic device in question can guide plasma propagation in the desired direction and thus control the spatial extent of wettability treatment, especially in more complex geometries.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10404-025-02793-9.

**Acknowledgements** Claire Douat would like to thank Dr. Emile Carbone (INRS, University of Quebec) for his help in the identification of the CN band and prof. Nadjib Semmar (GREMI, University of Orleans, France) for his help in the evaluation of the gas temperature. We kindly thank Eric Bourhis and ICMN laboratory (Orleans, France) who helped us with XPS surface analysis. The XPS analysis has been made possible thanks to the CERTeM 2020 Program, which received the financial support of the Regional Council Centre-Val de Loire (CERTeM 2020 201400096812) and is co-funded by the European Union through the European Regional Development Fund (ERDF 2017-EX002247).

**Author Contributions** Conceptualization: VG, CD, AS, SR. Investigation: VG, CD. Methodology: VG, CD, SD. Validation: VG, CD, AS, SR. Visualization: VG, CD. Writing - original draft: VG. Writing - review and editing: CD, AS, SR. Supervision: CD, AS, SR. Funding Acquisition: CD, AS, SR. Project administration: SR.

**Funding** Open access funding provided by Université d' Orleans. This project has received financial support from the Agence Nationale de la Recherche (ANR) through the LabEx VOLTAIRE (grant number ANR-10-LABX-100-01) and the research project ANR-21-CE50-0038-01, and from the CNRS through the 80|Prime program from the Mission pour les Initiatives Transverses et Interdisciplinaires (MITI).

**Data availability** Data for this article, including data points of contact angle measurements, XPS spectra are available at Zenodo repository at 10.5281/zenodo.13312215. Materials availability Not applicable.

**Code availability** Not applicable.

## Declarations

**Conflict of interest** There are no conflict of interest.

**Ethics approval and consent to participate** Not applicable.

**Consent for publication** Not applicable.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source,

provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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