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ABSTRACT: Glass capillaries are prepared with well-defined regions of tuneable wettability on the interior walls using an inexpensive and simple approach. A homogeneous layer of hydrophilic TiO2 nanoparticles is adsorbed on the capillary wall and chemically hydrophobized using octadecyltrihydrosilane (OTHS). The hydrophobic OTHS monolayer is then patterned by spatially-selective removal of the OTHS via TiO2-catalysed decomposition by ultraviolet irradiation. By patterning the capillaries with hydrophilic-hydrophobic rings, modulated penetration of a liquid (glycerol, in this study) can be achieved. For given wettability contrast, the penetration dynamics and equilibrium rise heights are very sensitive to the characteristic length-scale of the pattern, and may offer greater, time-dependent sampling control in fluidic devices.
Journal of Colloid and Interface Science 04/2013; · 3.07 Impact Factor
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ABSTRACT: Analytical technologies of ultrasmall volume liquid, in particular femtoliter to attoliter liquid, is essential for single-cell and single-molecule analysis, which is becoming highly important in biology and medical diagnosis. Nanofluidic chips will be a powerful tool to realize chemical processes for such a small volume sample. However, a technical challenge exists in fluidic control, which is femtoliter to attoliter liquid generation in air and handling for further chemical analysis. Integrating mechanical valves fabricated by MEMS (microelectric mechanical systems) technology into nanofluidic channels is difficult. Here, we propose a nonmechanical valve, which is a Laplace nanovalve. For this purpose, a nanopillar array was embedded in a nanochannel using a two-step electron beam lithography and dry-etching process. The nanostructure allowed precise wettability patterning with a resolution below 100 nm, which was difficult by photochemical wettability patterning due to the optical diffraction. The basic principle of the Laplace nanovalve was verified, and a 1.7 fL droplet (water in air) was successfully generated and handled for the first time.
Analytical Chemistry 12/2012; · 5.86 Impact Factor
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ABSTRACT: Maskless microplasma treatment of passivated surfaces has been developed for the micropatterning of materials surfaces. The micropatterned surfaces are used in the fabrication of arrays for protein and cell-based assays. The advantage of this micropatterning approach is that it can be easily integrated into current manufacturing practices and the resultant micropatterned surfaces used with existing life sciences techniques and instrumentation.
RSC Advances 01/2012; 2(31):12007-12010.
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Chemical Engineering & Technology 01/2012; 35(7):1. · 1.60 Impact Factor
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ABSTRACT: The dynamics of capillary-driven flow was studied for water and water–glycerol mixtures in open hydrophilic microchannels (embedded in a hydrophobic matrix). The position of the advancing meniscus was recorded as a function of time using high speed microscopy and compared with the Washburn equation. The square of the position of the liquid front increased linearly with time, as predicted by Washburn. For a channel of the same depth, irrespective of the shape of the channel cross-section (rectangular or curved), the liquid flow was faster with decreasing channel width. A modified Washburn equation, accounting for the different flow profile in the open, noncylindrical channels, was developed. The theoretical prediction was in good agreement with the experimental data for a no-slip boundary condition at the liquid–air interface.
09/2011;
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ABSTRACT: The formation and stability of drops in the presence of nanoparticles was studied in a microfluidic device to directly observe the early stages of Pickering emulsification (low interfacial coverage). We observed several key differences between oil droplet necking and rupture in aqueous phases of nanoparticles (methylated silica) and well-characterised surfactant systems. The presence of particles did not influence drop formation dynamics and thus the size of the drops generated. In addition, observations of in-channel drop stability shortly after formation (several milliseconds) indicated that particles in the aqueous phase slow film thinning processes, but do not prevent coalescence. In contrast, downstream collection and densification (at the microchannel outlet), showed that particle-stabilised drops do not coalesce for several weeks, above a critical particle concentration. The implications of our results for droplet microfluidics and our understanding of conventional emulsification systems are discussed.
Journal of Colloid and Interface Science 07/2011; 363(1):301-6. · 3.07 Impact Factor
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ABSTRACT: A rapid, high-precision method for localised plasma-treatment of bonded PDMS microchannels is demonstrated. Patterned electrodes were prepared by injection of molten gallium into preformed microchannel guides. The electrode guides were prepared without any additional fabrication steps compared to conventional microchannel fabrication. Alignment of the "injected" electrodes is precisely controlled by the photomask design, rather than positioning accuracy of alignment tools. Surface modification is detected using a fluorescent dye (Rhodamine B), revealing a well-defined micropattern with regions less than 100 µm along the length of the microchannel.
Lab on a Chip 10/2010; 11(3):541-4. · 5.67 Impact Factor
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ABSTRACT: A droplet of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, bmim.BF(4)) is immersed in an immiscible liquid (n-hexadecane) and electrowetted on a flat Teflon AF1600-coated ITO electrode. The static contact angle decreases significantly when voltage is applied between the droplet and the electrode: from 145 degrees down to 50 degrees (with DC voltage) and 15 degrees (with AC voltage). The electrowetting curves (contact angle versus voltage) are similar to the ones obtained in other solid/liquid/vapor and solid/liquid/liquid systems: symmetric with respect to zero voltage and correctly described by Young-Lippmann equation below saturation. The reversibility is excellent and contact angle hysteresis is minimal (approximately 2 degrees). The step size used in applying the DC voltage and the polarity of the voltage are unimportant. The saturation contact angle cannot be predicted with the simple zero-interfacial tension theory. Spreading (after applying a DC voltage) and retraction (after switching off the voltage) of the droplet is monitored. The base area of the droplet varies exponentially during wetting (exponential saturation) and dewetting (exponential decay). The characteristic time is 20 ms for spreading and 35 ms for retraction (such asymmetry is not observed with water-glycerol mixtures of a similar viscosity). The spreading kinetics (dynamic contact angle versus contact line speed) can be described by the hydrodynamic model (Voinov's equation) for small contact angles and by the molecular-kinetic model (Blake's equation) for large contact angles. The role of viscous and molecular dissipation follows the scheme outlined by Brochard-Wyart and de Gennes.
Journal of the American Chemical Society 06/2010; 132(24):8301-8. · 9.91 Impact Factor
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ABSTRACT: The influence of water on the electrowetting performance of the ionic liquid (IL) [BMIM][BF4] in contact with an ambient phase of hexadecane was studied. Electrowetting experiments on a fluoropolymer surface with IL concentrations between 0 and 99% enabled investigation of various electrowetting parameters, including the maximum range of accessible contact angles, the saturation contact angle (and voltage), and spreading dynamics. Experiments carried out using dc and ac potentials were successfully described with the Young−Lippmann equation prior to contact angle saturation. The saturation contact angles differed for ac and dc electrowetting, indicating distinct saturation mechanisms. The dilution of [BMIM][BF4] by water, within the range of concentrations studied, had no direct impact on the electrowetting behavior, aside from indirect effects due to altered viscosity and interfacial tension.
04/2010;
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Craig Priest
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ABSTRACT: Microfluidic channels in which multiple chemical and biological processes can be integrated into a single chip have provided a suitable platform for high throughput screening, chemical synthesis, detection, and alike. These microchips generally exhibit a homogeneous surface chemistry, which limits their functionality. Localized surface modification of microchannels can be challenging due to the nonplanar geometries involved. However, chip bonding remains the main hurdle, with many methods involving thermal or plasma treatment that, in most cases, neutralizes the desired chemical functionality. Postbonding modification of microchannels is subject to many limitations, some of which have been recently overcome. Novel techniques include solution-based modification using laminar or capillary flow, while conventional techniques such as photolithography remain popular. Nonetheless, new methods, including localized microplasma treatment, are emerging as effective postbonding alternatives. This Review focuses on postbonding methods for surface patterning of microchannels.
Biomicrofluidics 01/2010; 4(3):32206. · 3.37 Impact Factor
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ABSTRACT: The wettability of surfaces microstructured with square pillars was studied, where the static advancing contact angle on the planar surface was 72 degrees. We observed elevated advancing angles (up to 140 degrees) on these structures for droplets in the Wenzel state. No air was trapped in the structured surfaces beneath the liquid, ruling out the well-known Lotus leaf effect. Instead, we show that the apparent hydrophobicity is related to contact line pinning at the pillar edges, giving a strong dependence of wetting hysteresis on the fraction of the contact line pinned on pillars. Simulating the contact line pinning on these surfaces showed similar behavior to our measurements, revealing both strong pinning at the edges of the pillars as well as mechanistic details.
Langmuir 09/2009; 26(2):860-5. · 4.19 Impact Factor
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ABSTRACT: The wetting behavior of hydrophobic, microstructured surfaces containing arrays of pillars or holes has been investigated. The size of the surface features was fixed (20 microm), while their separation was varied to adjust the area fraction (0-80%). The wettability of structured surfaces for liquids resting in the Cassie state is strongly dependent on the continuity of the solid component. Microstructured square pillars and holes showed distinct, asymmetric wetting hysteresis, consistent with our previous observations on flat, chemically heterogeneous surfaces. Furthermore, clear trends for the magnitude of contact angle hysteresis versus area fraction for the two types of microstructured surfaces are evident. The pinning energy associated with these surface features is estimated.
Langmuir 04/2009; 25(10):5655-60. · 4.19 Impact Factor
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ABSTRACT: Exploiting microfluidic principles, the potential for chip-based multilayer assembly for the synthesis of polymer microcapsules was investigated. We demonstrate that continuous flow microfluidic multilayer synthesis is a fast, efficient, automated alternative to conventional batch synthesis. In this work, we dispersed liquid crystal (LC) molecules (organic phase) as monodisperse droplets in an aqueous continuous phase containing the primary polymer and a suitable surfactant. The primary polymer was coadsorbed with the surfactant at the organic/aqueous interface, stabilizing the LC droplets against coalescence and providing a template for subsequent polymer adsorption. As the droplet templates are transported through the microfluidic channel, the polymer-containing aqueous continuous phase is selectively withdrawn and replaced with rinse solution, and then with an alternative polymer solution. This selective withdrawal and infusion cycle was repeated to assemble polymer multilayers onto the emulsion droplets. The process was followed using fluorescence microscopy of the fluorescently-labelled polymers at the LC interface and of the flowing polymer solutions during the sequential rinse stages. Cross-linking of the multilayers and removal of the dispersed LC phase resulted in polymer capsules retaining the high monodispersity of the droplet templates. This microfluidic approach significantly reduces the multilayer formation time (to <2 min for 3-layer capsules) of well-defined capsules that are envisaged to have benefits in biomedical applications, including drug delivery and encapsulated biochemical reactions.
Lab on a Chip 01/2009; 8(12):2182-7. · 5.67 Impact Factor
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ABSTRACT: The velocity dependence of the dynamic contact angle for a glycerol-water mixture wetting two different chemically heterogeneous surfaces (mixed thiols on gold and partially methylated titania, 16 samples in all) was studied. The molecular kinetic theory (MKT) of wetting was used to interpret the dynamic contact angle data. The equilibrium displacement frequency ( K 0) was predominantly determined by the viscous contribution from the bulk liquid, with a minor contribution from the surface. The mean distance between surface sites (lambda) decreased with increasing work of adhesion. The contact line friction coefficient zeta 0 was found to vary exponentially with the work of adhesion, enabling the unit flow volume of the liquid to be obtained.
Langmuir 11/2008; 24(22):13007-12. · 4.19 Impact Factor
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ABSTRACT: Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been examined as a possible predictive tool for surface wettability. Heterogeneous surfaces were prepared with hydrophilic and hydrophobic regions of known surface coverage using self-assembled monolayers. The surface coverage of each component was then correlated with ToF-SIMS fragmentation of the hydrophobic and hydrophilic surface groups and static contact angle measurements. From these measurements, a clear relationship between the surface wettability and relative intensity of characteristic secondary ions was identified. Moreover, our results for planar surfaces can be extrapolated to predict the wettability of particulate samples for which direct contact angle measurements are not straightforward. The ability to infer particle wettability by ToF-SIMS is well suited to mineral characterization and in particular, the prediction of mineral flotation efficiencies.
Journal of Colloid and Interface Science 05/2008; 320(2):563-8. · 3.07 Impact Factor
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ABSTRACT: We have explored microfluidic solvent extraction as a tool for high-throughput
mineral separation. The microfluidic extraction efficiency for chromium, compared
with bulk extraction, is shown for a range of concentrations and extraction (or contact)
times. We also demonstrate selectivity for chromium from a model chromite
leach solution and discuss throughput considerations.
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences, San Diego; 01/2008
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ABSTRACT: Using the Wilhelmy plate technique, the role of chemical defects in hysteretic wetting behavior was investigated. The wetting and dewetting work differ significantly, depending on the defect energy (i.e., high or low energy with respect to the matrix). For one, or an array of high-energy defects, advancing measurements departed from equilibrium theory, while the receding data were in close agreement. Conversely for low-energy defects, only the receding measurements showed significant departure from theory. We propose that distinct wetting mechanisms for high- and low-energy defects explain the phenomenon of asymmetric hysteresis, where the advancing or receding contact angle deviates more strongly from the equilibrium angle.
Physical Review Letters 08/2007; 99(2):026103. · 7.37 Impact Factor
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ABSTRACT: Patterned self-assembled monolayers of functionalised alkane thiols were prepared on gold substrates, using UV-photolithography. Two alkane thiols, 11-mercaptoundecanoic acid (MUA) and a fluorinated decane thiol (FDT, CF3(CF2)7CH2CH2SH) were used to fabricate chemically structured surfaces which served as templates for zinc oxide (ZnO) crystallisation. When these patterns, containing high (MUA) and low (FDT) surface energy regions were exposed to a 10 mM zinc nitrate crystallising solution, nucleation occurred selectively on the low energy regions. After 90 min, hexagonal prisms had grown upright on these areas. The crystal growth is uniform with a crystal length of about 1 mum and a diameter between 50 and 100 nm. We attribute the selective growth to a combination of crystallographic frustration of the zinc ions on the high energy regions and an accumulation of hydroxide ions on the low energy regions.
Journal of Colloid and Interface Science 12/2006; 303(2):333-6. · 3.07 Impact Factor
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ABSTRACT: Microfluidic solvent extraction (SX) of metal ions from particle-laden aqueous solutions is demonstrated as an alternative to conventional solvent extraction for a system of industrial interest: extraction of Cu2+ using 2-hydroxy-5-nonylacetophenone. In the presence of silica nanoparticles, bulk SX suffers from prolonged phase separation and, for hydrophobic particles, the formation of particle-stabilised emulsions, which can be indefinitely stable, leading to significant losses of valuable materials to the emulsion phase. In contrast, non-dispersive microfluidic SX can process fluids containing high particle concentrations (e.g. 61 g/L, 80 nm hydrophilic silica and 5 g/L, and 13 nm moderately hydrophobic silica). The SX was operated continuously for more than 7 h without blockage or failure of the microfluidic chip, in part due to the very short residence time of the silica nanoparticles in the aqueous phase. The microfluidic method is also able to access extraction kinetics for particle-laden systems, which cannot be obtained otherwise due to delayed phase separation.Graphical abstractResearch Highlights►Solvent extraction (SX) of copper using microfluidic streams was studied ►Particle-laden aqueous phases were used as a non-ideal reference extraction ►Conventional (bulk) methods were hindered or arrested with particles present ►Phase separation times were effectively zero for microfluidic SX ►Extraction progress could be followed at short time-scales for microfluidic SX
International Journal of Mineral Processing 98:168-173. · 1.30 Impact Factor
