Nanoscale surface modifications to control capillary flow characteristics in PMMA microfluidic devices
© Mukhopadhyay et al; licensee Springer. 2011
Received: 30 October 2010
Accepted: 3 June 2011
Published: 3 June 2011
Polymethylmethacrylate (PMMA) microfluidic devices have been fabricated using a hot embossing technique to incorporate micro-pillar features on the bottom wall of the device which when combined with either a plasma treatment or the coating of a diamond-like carbon (DLC) film presents a range of surface modification profiles. Experimental results presented in detail the surface modifications in the form of distinct changes in the static water contact angle across a range from 44.3 to 81.2 when compared to pristine PMMA surfaces. Additionally, capillary flow of water (dyed to aid visualization) through the microfluidic devices was recorded and analyzed to provide comparison data between filling time of a microfluidic chamber and surface modification characteristics, including the effects of surface energy and surface roughness on the microfluidic flow. We have experimentally demonstrated that fluid flow and thus filling time for the microfluidic device was significantly faster for the device with surface modifications that resulted in a lower static contact angle, and also that the incorporation of micro-pillars into a fluidic device increases the filling time when compared to comparative devices.
In recent years, microfluidics has become an indispensable component of microelectromechanical systems (MEMS) technology [1–3], with polymer devices establishing a greater role in the development of disposable microfluidic systems . One such polymer is polymethylmethacrylate (PMMA) which is used in the fabrication of a wide variety of microfluidic devices [4, 5], from micro-reactors  to high aspect ratio microstructures , blood filters , and waveguide devices . Additionally, PMMA microfluidic systems may be fabricated using a wide range of techniques, including injection molding, hot embossing, laser photo-ablation, and X-ray lithography [3, 4, 6, 7, 9].
Passive capillary flow is an important consideration for disposable polymeric microfluidic devices [2, 10–12], where flow can be modified by adjusting the surface wettability or by incorporating surface roughness features on the interior surface of the microchannels. In literature, mainly three surface engineering strategies have been developed that can directly change nanoscale surface properties of polymer. First, fabricating desired surface features by various micro fabrication techniques, such as lithographic, hot-embossing, etc. Secondly, vacuum-based thin film coating techniques can be used to modify surface properties. Finally, polymer surface can be modified by physical (such as plasma) and chemical routes. Recent reports have shown that the surface wettability for polymer devices can be varied by plasma treatment [13–16] and the coating of diamond-like carbon (DLC) film [17, 18] on the microchannel surfaces with numerous simulations describing the effects of surface roughness on microfluidic flow [2, 19–22]. The plasma treatment and DLC coating on polymer for the surface modification have been well studied by other researchers. Chemical modification technique is also useful on PMMA surface for microfluidic applications . Also, pristine and UV-modified PMMA surfaces were used in microfluidic devices for cell transport applications . However, to the best of our knowledge, the effect of these surface modifications on microfluidic flow is not well understood. As such this study on the effects of microfluidic flow is essential to aid the design and fabrication processes of polymer microfluidic systems.
In this study, we have fabricated PMMA microchannels incorporating a micropillar array structure and subjected the channel surfaces to both plasma modification and DLC coatings to study the effects of surface modification on surface energy and surface chemistries. The capillary flow (in terms of capillary meniscus position and filling time of the devices) was recorded as a series of video files, which were subsequently analyzed to correlate the flow behavior of the surface modification system.
Fabrication of PMMA microchannels
An SU8 stamp-on-silicon-wafer was fabricated using an SF100 maskless photolithograph system (Intelligent Micro Patterning, LLC, USA) using an established SU8 processing method [5, 25, 26]. Briefly, SU8 50 was coated onto the silicon substrate using a spin coater at 1000 RPM to make the SU8 stamp, patterned following a soft bake by exposure to UV light at an intensity of 310 μW/cm2 for 25 s, and developed following post-exposure bake in EC solvent for 10 min, following which the stamp was hard baked. The PMMA channels were fabricated from the stamp using a hot embossing system (EVG520, EVG Group, Austria) [5–7, 9] operating at 125°C and 10 kN for 2 min. Finally, a direct bonding technique was used [5, 27] to seal the PMMA devices to a PMMA lid. The bonding temperature and pressure used were 90°C and 10 kN, respectively, for bonding time of 4 min.
Surface modifications on the PMMA microchannel surfaces
We utilized four different methods to modify the pristine PMMA surfaces following heat and pressure treatment by hot embossing, but prior to thermal bonding, to modify the surface properties and wettability of the devices. The methods can be described briefly as follows:
Air dielectric barrier discharge (DBD) processing
Pristine PMMA surfaces were modified using an air DBD [13–16] treatment, adjusting the lift length to 740 mm with a 20% ramp over five cycles. After the plasma treatment on the PMMA surface, the sample was stored in air for 48 h and the static water contact angle was measured as 58.1. After 5 days of that plasma treatment, the contact angle on the same plasma-treated surface was subsequently measured as 44.3.
Nitrogen plasma treatment
Pristine PMMA surfaces were treated using an N2 plasma in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system . The vacuum pressure in PECVD chamber was maintained as 5.2 × 10-6 Torr, and the sample was cleaned with Ar at 60 sccm with a working pressure of 1.5 × 10-2 Torr. The treatment time was 5 min for the gas mixture of Ar (5 sccm) and N2 (10 sccm) and the working pressure during deposition was maintained at 3.0 × 10-3 Torr.
Coating of hydrogenated amorphous carbon (a-C:H)
Pristine PMMA surfaces were coated with hydrogenated amorphous carbon (a-C:H) using a PECVD system . The vacuum pressure in PECVD chamber was maintained as 4.6 × 10-6 Torr, and the sample was cleaned with Ar at 60 sccm with a working pressure of 1.6 × 10-2 Torr. The deposition time was 5 min for the gas mixture of Ar (10 sccm) and C2H2 (20 sccm) with a working pressure of 7.7 × 10-3 Torr.
Coating of Si-doped hydrogenated amorphous carbon (a-C:H)
Pristine PMMA surfaces were coated with Si-doped a-C:H using a PECVD system . The vacuum pressure in PECVD chamber was maintained as 6.2 × 10-6 Torr, and the sample was cleaned with Ar at 60 sccm at a working pressure of 1.8 × 10-2 Torr. The deposition time was 5 min for the gas mixture of Ar (5 sccm), C2H2 (10 sccm) and TMS (5 sccm) in 8.9 × 10-3 Torr chamber pressure.
The surface of PMMA was not rinsed with IPA and DI water prior to contact angle analysis. However, rinsing PMMA surfaces by IPA and DI water may alter contact angles . Two different liquids were used to carry out the contact angle measurements: ultra-pure water (MilliQ®) and Ethylene Glycol (Sigma-Aldrich, Gillingham, UK). Static contact angle were measured (using CAM 2000, KSV Instrument Ltd., Helsinki, Finland) by the sessile-drop method at room temperature (approximately 25°C). A 5 μl droplet of the liquid was deposited on the surface of the sample, and immediately after stabilization, an image of the droplet was captured. The profile of the droplet was automatically fitted with the CAM 2000 software using a Young Laplace approach. At least ten readings were performed per sample type and the corresponding average values and standard deviations were recorded. We have studied the surface chemistry and roughness of the unmodified and modified PMMA surfaces by X-ray photo electron spectroscopy (XPS) and atomic force microscopy (AFM), respectively.
Fluidic measurements for the microfluidic devices
Fluid flow in the sealed devices was recorded using a CMOS camera capturing video at 25 frames per second (fps) which corresponded to a frame resolution of 0.04 s/frame. The air-water interface velocity was measured from the recorded video clips  and the air-water interface velocity and the filling time (time for the meniscus to travel from the inlet to outlet of any particular device) were calculated by measuring the interface position change over a corresponding time interval [2, 5]. The laboratory temperature and humidity at the time of recording were in the order of 25°C and 28%, respectively. The flow was visualized using amaranth dyed water, with 15 μl amount of working liquid volume dispensed to minimize the entrance effect in each system. The meniscus movements were measured along the center line of the chamber.
Results and discussion
Surface energy, surface chemistry, and surface roughness
Static contact angles of dyed water and ethylene glycol on different surfaces
Static contact angles (°)
Air DBD processed PMMA
Nitrogen plasma-treated PMMA
Undoped DLC-coated PMMA
Si-doped DLC-coated PMMA
72.6 for hydrogenated amorphous carbon (a-C:H) coated surface, 79.9 for pristine PMMA surface and 81.2 for Si-doped a-C: H coated surface;
44.3 for air DBD processed PMMA surface and 52.1 for Nitrogen plasma-treated PMMA surface.
The plasma treatment processes were shown to significantly reduce static contact angle from 79.9 to between 52.1 and 44.3, data that supports results presented in previous reports on DLC  and plasma-treated PMMA  surfaces. Additionally, in the literature, surface wettability (hydrophobicity or hydrophilicity) has been defined using the static water contact angle [29, 32–35] method, and as such the plasma treatment process has been shown to considerably increased surface wettability-a feature which could be used to modify microfluidic flow in PMMA microchannels. The thickness of both undoped and Si-doped DLC coatings was measured as 70 nm by surface stylus profilometer.
Theoretical background of capillary flow in rectangular microchannel
where ρ, G, H, h, and μ are density, gravitational acceleration, height of the liquid reservoir at the inlet, microchannel height, and viscosity of liquid, respectively. The surface tension term (γsa - γsl) can be calculated from Equation 3. γsa is the surface tension of the solid-air interface and γsl is the surface tension of the solid-liquid interface. In this study, 1st term in right-hand side of Equation 3 has been neglected as H ~ 0 for surface driven flow.
where, γlg is the water-air surface tension per unit area. θ2 is the water-top wall contact angle and θ1 is the contact angle of water on any other wall of the microchannel. The water meniscus will propagate in microchannel by capillary action if ΔP has positive value , and in our experiments, we have observed that the air-water meniscus movement was slower in the microchannel of higher contact angle, indicating that θ1 (surface wettability reduced) was increased while maintaining w, h, and θ2. For higher contact angles fluid flow is significantly slower than for low contact angles, a conclusion supported by Equation 4.
Effect of surface wettability on microfluidic flow of dyed water
The filling time is higher for the microchannel surfaces with higher static water contact angle for any category of devices;
In Figures 7 and 8, the meniscus movement was faster for plasma-treated surfaces at all positions in the microchannels than on the pristine and DLC-coated PMMA surfaces. A similar trend of meniscus movement was shown the variation in pillar dimensions;
The linear relationship was shown between pillar dimension (none, 100, 200, 300 m) and flow velocity in the microchamber;
The effect of pillar side length was more pronounced on the surfaces of higher static water contact angle.
We also compared our experimental results with analytical simulation using Equation 2. Excellent fitting were obtained for all data. We have determined the values of diffusion coefficient for each microfluidic flow on different categories of surfaces (Figure 1). The diffusion coefficient was higher for the microfluidic flow on the surface of lower static contact angle (Table 2). We have evaluated the surface tension term (γsa - γsl) from Equation 3 from the results of analytical simulation. Surface tension term (γsa - γsl) was directly proportional to diffusion coefficient (D). The surface tension term was higher for the plasma-treated surfaces than other surfaces (Table 2). Yang et al.  has also shown that the surface tension term was higher for the surface of lower contact angle.
The variation of static water contact angle in the microchannel structure has been shown to have a significant influence on the speed of microfluidic flow, indeed Sultana et al.  demonstrated the dependence of microfluidic flow on surface wettability, and observed that the flow rate decreased with the decrease of surface wettability  between static contact angles of 20 and 89°. As lower static water contact angle is a measurement of higher surface wettability [30, 33, 35, 36], so the meniscus movement was faster on the microchannel surface of with lower static contact angle . Especially, when Suk and Cho  observed, the meniscus movement became significantly slower when the hydrophobic patterns were created in polymeric microchannel.
In this paragraph, we attempted to correlate fluid flow characteristic with surface properties and device structures. The lower filling time on plasma-treated surfaces was due to the higher polar surface energy due to plasma treatment (Figure 3). The surface chemistry studies revealed higher oxygen content and hydroxyl species on DBD processed PMMA surfaces (Figure 4b) and that may be one of the reasons of higher speed of fluid flow than on pristine PMMA surface. The higher surface roughness on DBD processed PMMA (Figure 5) may be another reason of higher speed of water flow than that on pristine PMMA surface . The geometry of the microchannel can also play a significant effect on microfluidic flow. We have modified the speed of microfluidic flow by varying the surface wettability and designed surface roughness on the bottom wall of microchannel, and Saha et al.  theoretically predicted that the channel walls and pillars would have significant effects on fluid flow, with the contributions being 1-3% for the side walls, 5-13% for the pillars and 85-89% from the top and bottom walls. In our study, we modified the bottom and side wall surfaces to change the surface wettability (measured as static contact angle on the surface), however the surface wettability on the top wall was constant for each device. Since the surface area to volume ratio is very high in microchannels, the surface wettability has a significant effect on microfluidic flow, and as a result a small change of contact angle leads to a larger change in capillary forces, thus making a significant change in filling time (Figure 9).
Effect of micropillar side length on the microfluidic flow of dyed water
In a pressure-driven flow system, Wang et al.  demonstrated that the Poiseuille number increased with increasing size of the roughness elements, while the mass flow rate can be seen to decrease with the increasing Poiseuille number . So, the mass flow rate was lower with increasing size of surface roughness elements. In our experimental study on capillary flow, PMMA micropillars were used to enhance surface roughness of the system. Pillar height and spacing were constant but the side length was varied to study the effect of micropillar dimension on microfluidic flow. Figure 9 illustrates that the filling time of the capillary meniscus was higher in the microfluidic device integrated with micropillars of higher side length. So, the speed of the capillary meniscus was lower in the device integrated with higher side length micropillars.
Values of diffusion coefficients and surface tension parameters from analytical simulation on each position-time curve of all the microfluidic flow
Diffusion coefficient, D(ρm2 s-1)
γsa - γsl
Air DBD processed
100 μm pillar
200 μm pillar
300 μm pillar
Nitrogen plasma treated
100 μm pillar
200 μm pillar
300 μm pillar
100 μm pillar
200 μm pillar
300 μm pillar
Undoped DLC coated
100 μm pillar
200 μm pillar
300 μm pillar
Si-doped DLC coated
100 μm pillar
200 μm pillar
300 μm pillar
In this study, we have shown that the microfluidic flow behavior can be significantly varied by two simple ways; first, by the plasma treatment on PMMA microchannel; secondly, by the variation of designed surface roughness (micropillars). Comprehensive analysis of surface energy and surface chemistry studies revealed the reasons for the change of fluid flow behavior in microchannel. The static water contact angle on PMMA surfaces being reduced significantly by a plasma treatment processes. The polar surface energy was shown to be higher for the surfaces of lower static water contact angle and the oxygen content, hydro-carbon groups, and surface roughness were notably higher on DBD processed PMMA than for pristine PMMA. The pristine and modified surfaces can be classified into two different groupings of wettabilities, determined by static water contact angles, above 70 and near to 50°. The dyed water flow was faster on the surface of lower static contact angle due to higher wettability, and the effect of pillar side length shown to be more significant on the surface of higher static water contact angle. This type of surface engineering of any polymeric material can be widely used in variety of applications such as microfluidic and bio-engineering.
atomic force microscope
frames per second
plasma enhanced chemical vapor deposition
X-ray photo electron spectroscopy.
- Luo JK, Fu YQ, Le HR, Williams JA, Spearing SM, Milne WI: Diamond and diamond-like carbon MEMS. J Micromech Microeng 2007, 17: S147-S163. 10.1088/0960-1317/17/7/S12View ArticleGoogle Scholar
- Saha AA, Mitra SK: Effect of dynamic contact angle in a volume of fluid (VOF) model for a microfluidic capillary flow. J Colloid Interface Sci 2009, 339: 461–480. 10.1016/j.jcis.2009.07.071View ArticleGoogle Scholar
- Verpoorte E, Rooij NFD: Microfluidics Meets MEMS. Proc IEEE 2003, 91: 930–953. 10.1109/JPROC.2003.813570View ArticleGoogle Scholar
- Mathur A, Roy SS, Tweedie M, Mukhopadhyay S, Mitra SK, McLaughlin JA: Characterisation of PMMA microfluidic channels and devices fabricated by hot embossing and sealed by direct bonding. Curr Appl Phys 2009, 9: 1199–1202. 10.1016/j.cap.2009.01.007View ArticleGoogle Scholar
- Mukhopadhyay S, Roy SS, Mathur A, Tweedie M, McLaughlin JA: Experimental study on capillary flow through polymer microchannel bends for microfluidic applications. J Micromech Microeng 2010, 20: 055018–1-055018–6.View ArticleGoogle Scholar
- Becker H, Heim U: Hot embossing as a method for the fabrication of polymer high aspect ratio structures. Sens Actuators A 2000, 83: 130–135. 10.1016/S0924-4247(00)00296-XView ArticleGoogle Scholar
- Li JM, Liu C, Dai XD, Chen HH, Liang Y, Sun HL, Tian H, Ding XP: PMMA microfluidic devices with three-dimensional features for blood cell filtration. J Micromech Microeng 2008, 18: 095021–1-095021–7.Google Scholar
- Mathur A, Roy SS, McLaughlin JA: Transferring vertically aligned carbon nanotubes onto a polymeric substrate using a hot embossing technique for microfluidic applications. J R Soc Interface 2010, 7: 1129–1133. 10.1098/rsif.2009.0520View ArticleGoogle Scholar
- Datta P, Goettert J: Method for polymer hot embossing process development. Microsyst Technol 2007, 13: 265–270.View ArticleGoogle Scholar
- Ichikawa N, Hosokawa K, Maeda R: Interface motion of capillary-driven flow in rectangular microchannel. J Colloid Interface Sci 2004, 280: 155–164. 10.1016/j.jcis.2004.07.017View ArticleGoogle Scholar
- Saha AA, Mitra SK, Tweedie M, Roy S, McLaughlin JA: Experimental and numerical investigation of capillary flow in SU8 and PDMS microchannels with integrated pillars. Microfluid Nanofluid 2009, 7: 451–465. 10.1007/s10404-008-0395-0View ArticleGoogle Scholar
- Suk JW, Cho JH: Capillary flow control using hydrophobic patterns. J Micromech Microeng 2007, 17: N11-N15. 10.1088/0960-1317/17/4/N01View ArticleGoogle Scholar
- Cui NY, Upadhyay DJ, Anderson CA, Brown NMD: Study of the surface modification of a Nylon-6,6 film processed in an atmospheric pressure air dielectric barrier discharge. Surf Coat Technol 2005, 192: 94–100. 10.1016/j.surfcoat.2004.03.006View ArticleGoogle Scholar
- Cui NY, Upadhyay DJ, Anderson CA, Meenan BJ, Brown NMD: Surface oxidation of a Melinex 800 PET polymer material modified by an atmospheric dielectric barrier discharge studied using X-ray photoelectron spectroscopy and contact angle measurement. Appl Surf Sci 2007, 253: 3865–3871. 10.1016/j.apsusc.2006.08.008View ArticleGoogle Scholar
- Liu CZ, Wu JQ, Ren LQ, Tong J, Li JQ, Cui N, Brown NMD, Meenan BJ: Comparative study on the effect of RF and DBD plasma treatment on PTFE surface modification. Mater Chem Phys 2004, 85: 340–346. 10.1016/j.matchemphys.2004.01.026View ArticleGoogle Scholar
- Upadhyay DJ, Cui NY, Meenan BJ, Brown NMD: The effect of dielectric barrier discharge configuration on the surface modification of aromatic polymers. J Phys D 2005, 38: 922–929. 10.1088/0022-3727/38/6/022View ArticleGoogle Scholar
- Abbas GA, McLaughlin JA, Harkin-Jones E: A study of ta-C, a-C:H and Si-a:C:H thin films on polymer substrates as a gas barrier. Diam Relat Mater 2004, 13: 1342–1345. 10.1016/j.diamond.2003.10.084View ArticleGoogle Scholar
- Cuong NK, Tahara M, Yamauchi N, Sone T: Diamond-like carbon films deposited on polymers by plasma-enhanced chemical vapor deposition. Surf Coat Technol 2003, 174–175: 1024–1028.View ArticleGoogle Scholar
- Croce G, Agaro PD: Numerical simulation of roughness effect on microchannel heat transfer and pressure drop in laminar flow. J Phys D 2005, 38: 1518–1530. 10.1088/0022-3727/38/10/005View ArticleGoogle Scholar
- Gamrat G, Favre-Marinet M, Person SL, Baviere R, Ayela F: An experimental study and modelling of roughness effects on laminar flow in microchannels. J Fluid Mech 2008, 594: 399–423.View ArticleGoogle Scholar
- Hay K, Dragila M: Physics of fluid spreading on rough surfaces. Int J Numer Anal Mod 2008, 5: 85–92.Google Scholar
- Wang XQ, Yap C, Mujumdar AS: Effects of two-dimensional roughness in flow in microchannels. J Electron Packag 2005, 127: 357–361. 10.1115/1.1997164View ArticleGoogle Scholar
- Soper SA, Henry AC, Vaidya B, Galloway M, Wabuyele M, McCarley RL: Surface modification of polymer-based microfluidic devices. Anal Chim Acta 2002, 470: 87–99. 10.1016/S0003-2670(02)00356-2View ArticleGoogle Scholar
- Witek MA, Wei S, Vaidya B, Adams AA, Zhu L, Stryjewski W, McCarley RL, Soper SA: Cell transport via electromigration in polymer-based microfluidic devices. Lab Chip 2004, 4: 464–472. 10.1039/b317093dView ArticleGoogle Scholar
- Carlier J, Arscott S, Thomy V, Fourrier JC, Caron F, Camart JC, Druon C, Tabourier P: Integrated microfluidics based on multi-layered SU-8 for mass spectrometry analysis. J Micromech Microeng 2004, 14: 619–624. 10.1088/0960-1317/14/4/024View ArticleGoogle Scholar
- Sameoto D, Tsang SH, Foulds IG, Lee SW, Parameswaran M: Control of the out-of-plane curvature in SU-8 compliant microstructures by exposure dose and baking times. J Micromech Microeng 2007, 17: 1093–1098. 10.1088/0960-1317/17/5/032View ArticleGoogle Scholar
- Tsao CW, DeVoe DL: Bonding of thermoplastic polymer microfluidics. Microfluid Nanofluid 2009, 6: 1–16. 10.1007/s10404-008-0361-xView ArticleGoogle Scholar
- Wei S, Vaidya B, Patel AB, Soper SA, McCarley RL: Photochemically patterned poly(methyl methacrylate) surfaces used in the fabrication of microanalytical devices. J Phys Chem B 2005, 109: 16988–16996. 10.1021/jp051550sView ArticleGoogle Scholar
- Lim YT, Kim SJ, Yang H, Kim K: Controlling the hydrophilicity of microchannels with bonding adhesives containing surfactants. J Micromech Microeng 2006, 16: N9-N16. 10.1088/0960-1317/16/7/N01View ArticleGoogle Scholar
- Okpalugo TIT, Ogwu AA, Maguire PD, McLaughlin JAD: Platelet adhesion on silicon modified hydrogenated amorphous carbon films. Biomaterials 2004, 25: 239–245. 10.1016/S0142-9612(03)00494-0View ArticleGoogle Scholar
- Brown L, Koerner T, Horton JH, Oleschuk RD: Fabrication and characterization of poly(methylmethacrylate) microfluidic devices bonded using surface modifications and solvents. Lab Chip 2006, 6: 66–73. 10.1039/b512179eView ArticleGoogle Scholar
- Bhattacharya S, Datta A, Berg JM, Gangopadhyay S: Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength. J Microelectromech Syst 2005, 14: 590–597.View ArticleGoogle Scholar
- Liu YC, Lu DN: Surface energy and wettability of plasma-treated polyacrylonitrile fibres. Plasma Chem Plasma Process 2006, 26: 119–126. 10.1007/s11090-006-9005-7View ArticleGoogle Scholar
- Xu LC, Siedlecki CA: Effects of surface wettability and contact time on protein adhesion to biomaterial surafces. Biomaterials 2007, 28: 3273–3283. 10.1016/j.biomaterials.2007.03.032View ArticleGoogle Scholar
- Yang C, Leong KC: Influences of substrate wettability and liquid viscosity on isothermal spreading of liquid droplets on solid surfaces. Exp Fluids 2002, 33: 728–731.View ArticleGoogle Scholar
- Goebel MO, Bachmann J, Woche SK, Fischer WR, Horton R: Water potential and aggregate size effects on contact angle and surface energy. Soil Sci Soc Am J 2004, 68: 383–393. 10.2136/sssaj2004.0383View ArticleGoogle Scholar
- Janssen D, Palma RD, Verlaak S, Heremans P, Dehaen W: Static solvent contact angle measurements, surface free energy and wettability determination of various self-assembled monolayers on silicon dioxide. Thin Solid Films 2006, 515: 1433–1438. 10.1016/j.tsf.2006.04.006View ArticleGoogle Scholar
- Kwok SCH, Wang J, Chu PK: Surface energy, wettability, and blood compatibility phosphorus doped diamond-like carbon films. Diam Relat Mater 2005, 14: 78–85. 10.1016/j.diamond.2004.07.019View ArticleGoogle Scholar
- Suzer S, Argun A, Vatansever O, Aral O: XPS and water contact angle measurements on aged and corona-treated PP. J Appl Polym Sci 1999, 74: 1846–1850. 10.1002/(SICI)1097-4628(19991114)74:7<1846::AID-APP29>3.0.CO;2-BView ArticleGoogle Scholar
- Zan HW, Chou CW, Wang CH, Yen KH, Hwang JC: Pentacene patterning on Aluminum Nitride by water dipping. J Electrochemical Soc 2008, 155: J321-J325. 10.1149/1.2976894View ArticleGoogle Scholar
- Desimoni E, Casella GI, Salvi AM: XPS/XAES study of carbon fibres during thermal annealing under UHV conditions. Carbon 1992, 30: 521–526. 10.1016/0008-6223(92)90170-2View ArticleGoogle Scholar
- Hozumi A, Masuda T, Hayashi K, Sugimura H, Takai O, Kameyama T: Spatially defined surface modification of poly(methylmethacrylate) using 172 nm vacuum ultraviolet light. Langmuir 2002, 18: 9022–9027. 10.1021/la020478bView ArticleGoogle Scholar
- Pfleging W, Adamietz R, Bruckner HJ, Bruns M, Welle A: Laser-assisted modification of polymers for microfluidic, microoptics and cell culture applications. In Laser-based Micro- and Nanopackaging and Assembly: Proceedings of Photonics West: Lasers and Applications in Science and Technology, San Jose, Calif., January 20–25, 2007. Bellingham, WA.: SPIE; 2007. S.645907 (SPIE Proceedings Series; 6459) S.645907 (SPIE Proceedings Series; 6459)Google Scholar
- D'Sa RA, Burke GA, Meenan BJ: Protein adhesion and cell response on atmospheric pressure dielectric barrier discharge-modified polymer surfaces. Acta Biomater 2010, 6: 2609–2620. 10.1016/j.actbio.2010.01.015View ArticleGoogle Scholar
- Packham DE: Surface energy, surface topography and adhesion. Int J Adhesion Adhesives 2003, 23: 437–448. 10.1016/S0143-7496(03)00068-XView ArticleGoogle Scholar
- Waghmare PR, Mitra SK: On the derivation of pressure field distribution at the entrance of a rectangular capillary. J Fluids Eng 2010, 132: 054502–1-054502–4.View ArticleGoogle Scholar
- Waghmare PR, Mitra SK: Finite reservoir effect on capillary flow of microbead suspension in rectangular microchannels. J Colloid Interface Sci 2010, 351: 561–569. 10.1016/j.jcis.2010.08.039View ArticleGoogle Scholar
- Yang LJ, Yao TJ, Tai YC: The marching velocity of the capillary meniscus in a microchannel. J Micromech Microeng 2004, 14: 220–225. 10.1088/0960-1317/14/2/008View ArticleGoogle Scholar
- Blanco-Gomez G, Glidle A, Flendrig LM, Cooper JM: Integration of low-power microfluidic pumps with biosensors within a laboratory-on-a-chip device. Anal Chem 2009, 81: 1365–1370. 10.1021/ac802006dView ArticleGoogle Scholar
- Sultana S, Matsui J, Mitsuishi M, Miyashita T: Flow behavior in surface-modified microchannels with polymer nanosheets. Thin Solid Films 2009, 518: 606–609. 10.1016/j.tsf.2009.07.048View ArticleGoogle Scholar
- Park SJ, Chung S, Bang HW, Chung C, Han DC, Chang JK: 2nd Annual International IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology, May 2–4, 2002. Madison, Wisconsin, USA; 2002:565.Google Scholar
- Rawool AS, Mitra SK, Kandlikar SG: Numerical simulation of flow through microchannels with designed roughness. Microfluid Nanofluid 2006, 2: 215–221. 10.1007/s10404-005-0064-5View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.