Wettability alteration in thiolene-based polymer microﬂuidics surface characterization and advanced

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Wettability alteration in thiolene-based

polymer microﬂuidics:

surface characterization and advanced

fabrication techniques

Mahtab Masouminia,†Kari Dalnoki-Veress,‡and Benzhong Zhao∗,†

†Department of Civil Engineering, McMaster University, Hamilton, Ontario L8S 4L8,

Canada

‡Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4L8,

Canada

E-mail: robinzhao@mcmaster.ca

Abstract

Wettability plays a signiﬁcant role in controlling multiphase ﬂow in porous media for

many industrial applications, including geologic carbon dioxide sequestration, enhanced

oil recovery, and fuel cells. Microﬂuidics is a powerful tool to study the complexities

of interfacial phenomena involved in multiphase ﬂow in well-controlled geometries. Re-

cently, the thiolene-based polymer called NOA81 emerged as an ideal material in the

fabrication of microﬂuidic devices, since it combines the versatility of conventional soft

photolithography with a wide range of achievable wettability conditions. Speciﬁcally,

the wettability of NOA81 can be continuously tuned through exposure to high-energy

UV. Despite its growing popularity, the exact physical and chemical mechanisms behind

the wettability alteration have not been fully characterized.

arXiv:2210.01843v2 [cond-mat.soft] 6 Oct 2022

Here, we apply diﬀerent characterization techniques, including contact angle mea-

surements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM)

to investigate the impact of high-energy UV on the chemical and physical proper-

ties of NOA81 surfaces. We ﬁnd that high-energy UV exposure increases the oxygen-

containing polar functional groups, which enhances the surface energy and hydrophilic-

ity of NOA81. Additionally, our AFM measurements show that spin-coated NOA81 sur-

faces have a roughness less than a nanometer, which is further reduced after high-energy

UV irradiation. Lastly, we advance the state-of-the-art of NOA81-based microﬂuidic

systems by creating i) a 2D surface with controlled wettability gradient and ii) a 3D

column packed with monodisperse NOA81 beads of controlled size and wettability.

Introduction

Fluid-ﬂuid displacement in small, conﬁned geometries is strongly inﬂuenced by the relative

aﬃnity of the surrounding solid for the diﬀerent ﬂuids (i.e., wettability).1–3 Wettability at

the small-scale has important implications in a variety of large-scale natural and industrial

processes, including vadose zone hydrology,4–6 enhanced oil recovery,7,8 geologic carbon and

hydrogen storage,9,10 and electrochemical energy conversion and storage.11,12 Microﬂuidic

devices oﬀer a powerful experimental platform to study the impact of wettability on ﬂuid-

ﬂuid displacement, since they allow for direct visualization of the ﬂuid interfaces and they

can be fabricated with controllable geometries.13–16

Diﬀerent techniques have been introduced to tune the wettability condition of microﬂuidic

experiments, which include the use of diﬀerent ﬂuid-ﬂuid pairs,17 chemical vapor deposition

(CVD) or liquid solution deposition of silane molecules,18,19 oxidization of polydimethyl-

siloxane (PDMS) surfaces via either corona discharge20 or oxygen plasma21 treatment, and

coating PDMS surfaces with a sol-gel layer that is functionalized with ﬂuorinated and pho-

toreactive silanes.22,23 Recently, the polymer called NOA81 (Norland Products, USA) has

emerged as an alternative material for fabricating microﬂuidic devices with controlled wetta-

bility conditions.24–26 NOA81 is a thiolene-based photocurable resin that enables patterning

of submicron-size features via soft imprint lithography.27 The wettability of NOA81 surfaces

can be tuned via exposure to high-energy UV irradiation. In addition to the many posi-

tive attributes including solvent resistance, biocompatibility, and high elastic modulus,28,29

NOA81 oﬀers the following advantages when it comes to wettability alteration: (i) its wet-

tability can be continuously tuned and controlled by varying the duration of high-energy

UV exposure.24 Speciﬁcally, the wettability as measured by the contact angle θof water in

silicone oil varies over a wide range (θ= 7◦–120◦).30 This wettability alteration is especially

desirable since both water and silicone oil are commonly used and well characterized analog

liquids in studies of capillarity and interfacial phenomena. (ii) the change in wettability is

stable over a timescale of many days.24

Despite the growing popularity of NOA81 in microﬂuidic studies (e.g.31–36), the physical

and chemical mechanisms behind the UV-induced wettability alteration have not been char-

acterized. Our work aims to ﬁll this knowledge gap. To this end, we ﬁrst create a highly

uniform NOA81 thin ﬁlm via spin coating on a silicon wafer. We then employ characteriza-

tion techniques including contact angle, X-ray photoelectron spectroscopy (XPS) and atomic

force microscopy (AFM) measurements to investigate changes to the NOA81 surface as a re-

sult of high-energy UV irradiation. Our analyses show that wettability alteration on NOA81

surfaces arise as a result of the emergence of polar, oxygen-containing functional groups

after high-energy UV exposure. Finally, we extend the potential use cases for NOA81 by

introducing procedures to generate (i) NOA81 surfaces with controlled wettability gradients

and (ii) monodisperse NOA81 beads with controlled size and wettability.

Surface fabrication

A survey of the literature shows that existing NOA81 surfaces in microﬂuidics applications

are fabricated via replica molding. In this method, NOA81 is sandwiched between a ﬂat

surface and a mold (typically made of PDMS) in the negative shape of the desired micro-

pattern before the NOA81 is cured.24–27,30 To generate a ﬂat NOA81 surface via replica

molding, we ﬁrst fabricate a ﬂat PDMS (Sylgard 184, Dow Corning, USA) substrate that

is cured on a silicon wafer. We then sandwich a drop of NOA81 between a silicon wafer

and the ﬂat PDMS substrate, separated by 100 µm thick precision shims. After curing the

NOA81 with 365 nm UV light for 10 s, we peel oﬀ the PDMS substrate to reveal the NOA81

surface (Fig. 1A). We expose the NOA81 surface to UV for an additional 20 s after PDMS

removal to cure the ultra thin superﬁcial layer of NOA81.27 We measure the topography and

roughness of the NOA81 surface with an atomic force microscope (MultiMode 8-HR, Bruker,

USA) with a scan area of 15 ×15 µm2. We ﬁnd that the NOA81 surface made by replica

molding has a root mean square (RMS) roughness of ∼1.32 nm, with a correlation length of

∼0.95 µm (Fig. 1C).

We then create a thin ﬁlm of NOA81 on a silicon wafer by spin-coating. The silicon

wafer is ﬁrst treated in an oxygen plasma asher for 1 min (Model PT7150, Bio-Rad, USA) —

NOA81 thin ﬁlms spin-coated on untreated silicon wafers are unstable at ambient tempera-

ture (20 ◦C) and they undergo spinodal dewetting. We achieve a stable NOA81 thin ﬁlm at

spin speed of 4000 rpm for 30 s, which is then cured with 365 nm UV light for 10 s (Fig. 1B).

The thickness of the cured NOA81 thin ﬁlm is ∼7.5 µm as measured by ellipsometry (Model

M-2000, J. A. Woolam, USA). AFM measurement of the spin-coated NOA81 surface reveals

that it has an RMS roughness of ∼0.59 nm, with a correlation length of ∼0.6 µm (Fig. 1D).

Therefore, spin-coating generates a smoother NOA81 surface at the nanoscale compared to

replica molding, and we use the spin-coated NOA81 surface in charaterizations discussed in

subsequent sections.

Figure 1: (A) Thin ﬁlm of NOA81 cured on an 100 ×100 silicon wafer created by PDMS replica

molding. The thickness of the NOA81 is 100 µm. (B) Thin ﬁlm of NOA81 cured on an 100 ×100

silicon wafer created by spin coating. The thickness of the NOA81 is ∼7.5 µm. (C) AFM

surface roughness measurements of the NOA81 ﬁlm created by PDMS replica molding. (D)

AFM surface roughness measurements of the NOA81 ﬁlm created by spin coating. Both

methods generate smooth coatings with roughness less than 14 nm, though spin coating

attains a signiﬁcantly more homogeneous ﬁlm.

Surface property change due to high-energy UV exposure

Contact angle characterization

NOA81 surface is known to become more hydrophilic after exposure to high-energy UV

irradiation. Levaché et al.24 characterized the UV-induced contact angle change in NOA81

surface made by replica molding for water-in-air and water-in-hexadecane oil systems. Here,

we treat the spin-coated NOA81 surfaces with high-energy UV for diﬀerent durations in a

UV-ozone cleaner (Model T0606B, UVOCS, USA), which generates UV emissions in the

185 and 254 nm range. We characterize the wettability of the surfaces by measuring the

static advancing contact angle of water using a contact angle goniometer immediately after

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摘要：

Wettabilityalterationinthiolene-basedpolymermicrouidics:surfacecharacterizationandadvancedfabricationtechniquesMahtabMasouminia,yKariDalnoki-Veress,zandBenzhongZhao,yyDepartmentofCivilEngineering,McMasterUniversity,Hamilton,OntarioL8S4L8,CanadazDepartmentofPhysics&Astronomy,McMasterUniversity,Hami...

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