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Super-Hydrophobic Surfaces as an On-chip Microfluidic Toolkit for Total Droplet Control Mark C. Draper, Colin Roger Crick, Viktorija Orlickaite, Vladimir A. Turek, Ivan Paul Parkin, and Joshua B. Edel Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac303786s • Publication Date (Web): 29 Apr 2013 Downloaded from http://pubs.acs.org on May 14, 2013

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Analytical Chemistry

Super-Hydrophobic Surfaces as an On-chip Microfluidic Toolkit for Total Droplet Control Mark C. Draper,[a] Colin R. Crick,[b] Viktorija Orlickaite,[b] Vladimir A. Turek,[a] Ivan P. Parkin,[b] and Joshua B. Edel[a][*] [*][a]

M. C. Draper, V. A. Turek, Dr. J. B. Edel.

Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ (UK)

[b]

Dr. C. R. Crick, V. Orlickaite, Prof. I. P. Parkin

Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ (UK)

E-mail: [email protected]

Abstract: We propose and outline a novel technique designed to utilise the unique surface repulsion present between aqueous droplets and customisable superhydrophobic surfaces for the on-chip spatial and temporal manipulation of droplets within inline continuous flow architecture. Through the integration of carefully designed and pre-patterned superhydrophobic surfaces into polymer microfluidic chipsets it is possible to take advantage of this enhanced surface repulsion to passively manipulate droplets on the microscale for a wide range of droplet interactions, including but not limited to acceleration, deceleration, merging and path control.

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Introduction Superhydrophobic surfaces (SHPSs) have recently gained significant interest due a number of potential applications across a vast range of fields and industries ranging from self cleaning window coatings[1] to surface enhanced bioanalytical techniques.[2] SHPSs are characterizable through measurement of the contact angle, Ѳc, produced between an aqueous water droplet in contact with the surface. These angles are typically defined to be greater than 150˚ with a roll off angle of less than 10˚.[3-5] An example of this can be seen in (fig. 1(a)) where a droplet deposited on a SHPS made from high-temperature polydimethylsiloxane (PDMS) gives a contact angle of 152˚ compared to a droplet on an untreated borosilicate glass slide with a Ѳc = 93˚. This relatively simple phenomenon has attracted much attention due to the relative simplicity in utilizing the inherent superhydrophobicity of SHPSs to achieve precise fluid control on both the large and micron scale. For example, Mertaniemi et al demonstrated two-dimensional control over aqueous droplets and fluids for unconfined macroscale environments using a deep reactive ion etching fabricated (DRIE) ‘Nanograss’[6] SHPS. Whilst similar work utilizing multiple polymer coatings such as poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), polyethylene (PE)[7] and PDMS[8] has also been show to be equally effective. Although these applications hold great promise, little work has been performed on incorporating SHPSs into flow-based applications confined within an enclosed two phase microfluidic system. There are a number of potential advantages in doing this, namely the passive manipulation of analytes, the spatial reduction of on-chip processes such as merging and mixing, and the potential to guide and confine well defined volumes of fluids (e.g. microdroplets). Furthermore, the ability to minimize the surface interaction between the fluid and microfluidic channel walls results in significant analytical advantages when it comes to fouling or non-specific surface adsorption. Following on from this, Micro- and nanostructured channel surfaces have previously been shown, both theoretically [9] and experimentally, to significantly affect both droplet behaviour in open and enclosed channel environments. Joonwon et al[10] demonstrated that the increase in contact angle associated with increasing density of micro and nanostructures also led to a linear reduction in the droplet sliding angle, suggesting a reduction in the level of friction, and subsequently flow resistance, One area of microfluidics that could significantly benefit from the incorporation of SHPS is droplet based and multiphase fluidics. Through the confluence of two immiscible laminar fluid flows at a suitable “T” or “flow focusing” junction, regular droplets can be readily produced[11-12] and utilised for complex chemical and biological experiments.[13] In an idealised system, droplet manipulation techniques such as droplet merging,[14] mixing,[15] path and velocity control[16] can all be managed within continuous and in-line channel architecture. There are currently several alternative on-chip techniques being employed throughout the literature to


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achieve the previously outlined droplet manipulations.These methods primarily revolve around the incorporation of additional structures and channels, such as Niu and colleagues use of merging “pillars” and “chambers” for forced droplet merging.[14] Additional methods such as size dependant[17] and pillar based electro-coalescence[18,19] have also been shown to be effective merging solutions, although such approaches require the incorporation of complex operational and fabrication considerations to the devices, namely the introduction of planar electrodes to the chip architecture.

In this manuscript we build on these aims and applications by proposing a novel strategy for the patterning and fabrication of SHPSs for integration into microfluidic systems. This opens the door to such surfaces being used for all forms of unit droplet operations (e.g. merging, velocity, spatial and temporal control) without the need of pillars, or even electrodes.

Experimental Section Superhydrophobic surface design and fabrication Departing from some of the more complicated SHPS fabrication techniques listed above, Crick et al have previously demonstrated a reliable thermophoretic aerosol assisted chemical vapour deposition (AACVD) technique for the application of an aerated PDMS mixture to a range of substrates.[20] In comparison to conventional PDMS structures, the thermophoretically deposited PDMS structure, shown in (fig 1(b)), displays a micrometer scale lattice structure with a surface roughness sufficient to induce the superhydrophobic Lotus effect.[21] Due to the high temperatures (≥ 300 ˚C) required in the AACVD process, conventional photolithographic techniques utilised for the tailored patterning of substrates could not be applied. To circumvent the incompatibility between conventional photolithography and the high temperatures employed in the deposition phase, a double lift off process similar to that introduced by Song et al[22] was employed. In brief, this involved the initial patterning of our AutoCAD designed masks onto glass slides in photoresist before coating them with gold via an Au sputtering process and subsequently performing an initial lift-off in acetone to leave a negative imprint of our design on the slide. With the Au patterned portions capable of withstanding the high temperatures involved in the AACVD process, the slides were then completely coated with the PDMS microlattice surface. To form our chosen superhydrophobic PDMS pattern, the slides were then immersed in a solution of aqua regia, thus dissolving the Au sections and allowing lift off of the undesired PDMS portions forming the patterns. Examples of patterns generated are shown in (fig 1(c)-(e)) with an outline of the patterning process, step-by-step, given in the ESI. In all cases the SHPS has thickness of 4 µm with width and length ranging between 0.2 – 22



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mm and 1 – 10 cm respectively. These features were embedded into microfluidic channels using conventional PDMS casting techniques with feature sizes ranging from 0.05 – 22 mm at a constant height of 50 µM. All chipsets contain the same flow focussing droplet creation junction, allowing selectivity of droplet throughputs in the range of 0 – 10 Hz dependent upon the design and level of surfactant

used

with

a

corresponding

droplet

size

in

the

range

5

–

200

nL.

Chip design and Fabrication In all experiments, PDMS microchannels were fabricated through standard photolithographic methods.[23] Briefly, channel architecture was designed using AutoCAD before being transferred to an acetate mask (Circuit Graphics) and used to expose a silicon wafer coated with negative SU-8 photoresist (Microchem) with UV light. UV crosslinked sections were subsequently removed via immersion in EC solvent to leave the desired channel architecture on the silicon surface. PDMS (Sylgard® 184 Elastomer) was subsequently cast on top of the mask and cured at 60 ˚C for 6 hours. Before bonding to the pre-coated superhydrophobically patterned surface, inlets and outlets were cut using 1 mm disposable biopsy punches (KAI Medical). The Au masks used for the SHPS patterning were created through a double lift-off process initially involving the spin-coating of positive AZ4562 resist (Microchemicals) onto a slide pre-cleaned in Piranha solution (4 parts sulfuric acid to 1 part hydrogen peroxide). (Caution: Piranha etch solution reacts violently with organic materials and should be handled with extreme caution.) Following this the photoresist was crosslinked via exposure to UV light before being developed by immersion in AZ400K solvent. Upon development the slide was then sputtered with gold before the sections coated with AZ4562 photoresist were removed through sonication in Acetone. The SHPS was applied through Aerosol Assisted CVD of Sylgard 184 to slides selectively coated with a photoresist/ gold pattern in a protocol similar to above. Coating of the SHPS involved a solution of PDMS (Sylgard® 184) (0.70 g) dissolved in chloroform (70 mL).[21] The solution was rapid stirred for 5 min and was used immediately. The AACVD depositions were carried out in a cold-walled horizontal-bed CVD reactor. The reactor contained a top and bottom glass plates, both coated with a 50 nm layer of SiO2 (dimensions: 145 × 45 × 5 mm) supplied by Pilkington NSG. A carbon block on which the bottom plate was placed heated the CVD reactor. The top plate was positioned 8 mm above and parallel to the bottom plate, the complete assembly was enclosed within a quartz tube. Slides were attached to the onto the top plate by steel wire, and were placed 5 mm away from the reactor inlet to ensure conformal coverage of all portions (shown in (ESI1)). The precursor aerosol[20] was generated using an


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ultrasonic humidifier (PIFCO HEALTH), with an operating frequency of 40 kHz and 25 W of power. The generated aerosol was moved to the reactor using a nitrogen gas flow through polytetrafluoroethylene and glass tubing, where it entered between the top and bottom plates. The reactor waste gas left via an exhaust. The nitrogen flow (1.0 L/min) carried the vapour from the flask until all liquid was gone, typically taking 45 min per deposition. The reactor was switched off and allowed to cool to room temperature, the nitrogen flow was left on for a further 10 min. The cooled slides were removed and handled in air. The reactor temperature, as measured by a thermocouple in the carbon heater block was maintained at 390 °C. Upon completion of the coating process, the slides were subsequently placed into a freshly prepared solution of aqua regia (1 part nitric acid to 3 parts hydrochloric acid) to dissolve the gold coated portions and lift of the unwanted PDMS sections. (Caution: Aqua Regia is a hazardous chemical and should be handled with extreme

caution.) Depositions were also carried out using substrates at room temperature for the SHPS track work. This was performed by running the experiment as previously, but the microscope was repositioned to be adjacent to the exhaust outlet, and the flow rate was raised to 1.5 L/min. The centre of the chip template was aligned and secured 10 mm away from the centre of the outlet. After all precursor solution was carried over the heated nitrogen flow was left on for a further 10 mins, the experimental configuration is outlined in the ESI (ESI1). Once coated these slides were immersed in acetone, no Au mask needed at low temperatures, to leave the desired SHPS sections. Following this, the slide was then cleaned in DI and dried with nitrogen gas. The slide was then bonded to the pre-prepared PDMS sections through being brought into contact after application of an air plasma for a duration of 60 seconds. Where applicable, PTFE tubing was subsequently inserted and sealed to the chip using KwikCast® epoxy (WPI). Nanoparticle Doped “Dye” Solution 16 nm 12-mercaptododecanoic acid functionalized gold nanoparticles (Au NP’s) produced as detailed elsewhere[24] were used instead of standard food colorant to dope the aqueous phase due to an observed reduction in contact angle upon doping of DI water solution with food colorant. In contrast, adding Au NP’s gave a better contrast ratio for videos and provided a contact angle consistent with that expected for water, as seen in the ESI. FC40 fluorinated oil was used throughout for the oil phase with an additional 2 % home synthesized surfactant[23] added to improve droplet regularity.

Results and discussion Due to the anticipated increase in surface tension experienced by a droplet moving onto a SHPS



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enclosed within such microfluidic channels, it is expected that a reduction in droplet velocity will be observed. This will occur due to the droplet striving to stay on the energetically favourable glass bottom portion of the channel before an increase in pressure behind the droplet forces it over the potential energy barrier created by the SHPS. Initial experiments were proposed to test the above hypothesis through incorporation of a SHPS section into the microfluidic channel shown in (fig. 1 (c)(i)) alongside multiple frames depicting the droplets journey through the chip (fig. 1 (c)(ii)). Briefly, a droplet (5 nl < Volume < 50 nl) is produced at the flow focussing junction before travelling down the microfluidic channel to encounter the 4 µm thick SHPS. A velocity-time plot of the data collected is displayed in (fig. 2 (b)(i)-(ii)) demonstrating temporal manipulation of consecutive (20+) droplet trajectories through the chipset. Comparative data for a droplet encountering a shorter and slightly higher surface (7 µm thick) is shown for comparison in (fig. 2 (b)(iii)-(iv), demonstrating the higher levels of deceleration and acceleration which can be achieved through manipulation of the surface height. Initially, the droplets move with a linear velocity through the microchannel before encountering the SHPS where it can be seen to slow dramatically whilst traversing the boundary. This is highlighted in fig. 3 where different total flowrates (Tf) and water fractions (Wf) are used. The total flow rate is defined as the summation of the aqueous, Fw, and oil, Fo, flow rates whilst Wf = Fw/ Tf.[23] In fig. 3a the Wf was varied between 0.05 and 0.25 and the Tf was held constant at 4 µl/min. Conversly in fig. 3b the Wf was held constant at 0.15 and the Tf varied between 2 – 8 µl/min. This was performed to characterize the droplet interaction with the SHPS.

It is worth noting that after the droplets moves past the SHPS boundary the droplet is then observed to accelerate to a higher constant velocity down the length of the channel. For all surface lengths, heights, flowrates and water fractions investigated, a 12.51 +/- 2.67 % increase in velocity was observed. Since we theorize that an increase in the droplet contact angle occurs once the droplet moves onto the SHPS, it can be expected that there is less surface friction acting on the droplet and, as such, it travels faster across such surfaces in comparison to normal PDMS-Glass chip microchannels. The structural density of the SHPS is kept consistent throughout but as shown by Joonwon et al,[10] further modification to this variable could lead to an even higher maximum velocity. As can be seen within the ESI (ESI4) the initial deceleration at the boundary is found to be linearly dependent upon both the water fraction and total flowrate facilitating a temporary reduction in the droplets velocity in the range 27.9% to 57.14 %. Through specific tailoring of the water fraction, flowrate alongside further investigation and manipulation of the SHPS structure density, it is expected that this range can be increased further. Upon reaching the end of the SHPS the droplet moves back onto the uncoated glass portion of the channel. As can be observed clearly


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in (fig. 2 (b)(iv)) and throughout (fig. 3) this causes the velocity to rise sharply before dropping back down to match the initial flow rate of the system. Quantitatively, acceleration due to movement off the end of the SHPS is found to be less dependent on water fraction and flowrate where an acceleration of approximately 10.96 % (σ = 1.64) of the pre-boundary velocity value is observed. As can be inferred throughout (fig. 3 (a)-(b)(i-iii)), the observed acceleration across the SHPS only continues until the droplet has reached the higher flowrate associated with the new frictional force incident upon it, at which point it maintains this higher velocity to the end of the surface. Since the droplet subsequently regresses back to a lower total flowrate, we can deduce that the change in frictional force is temporary and confined to the period in which the droplet is on the SHPS. As such, the careful design and patterning of SHPSs and their incorporation into enclosed microfluidic channels allows for creative manipulation of this force opening up the potential for the specific fine tuning of droplet velocity according to the needs of the system and the particular detection methods being employed.

The ability to manipulate droplet speed within systems has many potential avenues of interest. One such application involves the specific tailoring of SHPSs to facilitate controlled collisions between sequential droplets in a system leading to enhanced merging. One of the most common on-chip droplet manipulation techniques required in multiprocess systems is the ability to merge droplets to allow the addition of reagents to well defined sample volumes. As outlined in (fig. 1 (d)(i-ii)) our approach to this problem revolves primarily around the incorporation of a SHPS into the channel. As can be seen in (fig. 4 (b)), the dimensions of the surface are selected such that the SHPS is smaller than the dimensions of the microfluidic channel, with the gap between the SHPS and side wall of the channel being less than the radius of the droplets being merged. Once the droplet encounters the surface, it is energetically favourable for it to partially move into the gap between the surface and channel wall, as can be seen in (fig. 4 (a)), yet is too large to completely avoid the SHPS via this corridor. In partially moving into the corridor, the droplet shape is altered to the extent that the surface tension is reduced at the point of collision with the advancing droplet, thus allowing merging to occur under the condition that the gap between the side wall of the channel and SHPS is smaller than the radius of the droplet. When the gap exceeds this limitation, it is then possible for the droplets morphology to change such that it can travel past the SHPS pattern in this space before the next droplet is incident. Conversely, when the droplet diameter begins to approximate the channel width merging is no longer observed as the energy associated with the pressure build up behind the droplet becomes sufficient to overcome the potential energy barrier preventing the droplet from moving onto the SHPS.[15] For our current system this is found to occur at Wf greater than 0.25 and



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an example can be seen with the SI showing no significant velocity manipulation. (fig. 4 (a)) Outlines the merging process, for two droplets with volume of approximately 50 nl (2 Hz production rate), through 8 sequential frames taken from a video sequence. As frames 1-3 outline, in the first 200 ms the initial droplet, D1, approaches the SHPS and moves away from the surface towards the channel wall in an attempt to reduce the interfacial tension on the droplet surface. If the trapped droplet width is less than the total channel width, the droplet will remain at this point indefinitely. Upon arrival of a subsequent aqueous droplet, D2, coalescence will occur, as seen in frame 4. Due to the presence of the SHPS, there is an increase in the interfacial tension between molecules on the surface of stationary droplet promoting a change in the droplets morphology as the droplet moves to reduce this tension. This change in morphology produces a droplet shape more condusive for coalescence to occur between the stationary and incident droplet.[25] After merging the combined momentum of the droplets is sufficiently large for them to traverse the remaining SHPS and continue along the channel. (fig. 4 (c)) shows a velocity-time profile of two merging droplets, D1 and D2. The timeframe for merging under these conditions can be seen within the frames and occurs approximately 300 ms after trapping of the initial droplet. Merging was found to be consistent for multiple water fractions in the range 0.0625 < Wf < 0.12. Additionally, it was found that maximum total flowrate of the system was 9 µL/min, above this rate droplets would not consistently merge resulting in droplets being pushed over the surface instead of the expected merging.

Aside from velocity control, and merging, it is also possible to take advantage of the same dropletto-surface repulsion responsible for droplet deceleration and apply it to droplet path control. As the previous sections demonstrated, through application of SHPSs we have been able to demonstrate temporal control over movement of droplets in the x-plane; however, we are also able to control movement in the y-plane opening up the potential to significantly enhance droplet mixing alongside the multiplexing of on-chip processes. Building on the work by Mertaniemi et al,

[6]

whereby

superhydrophobic tracks were patterned into a Nanograss coated silicon wafer to provide an energetically favourable path for gravity driven droplets to follow, we pattern predesigned superhydrophobic PDMS tracks onto clean glass slides and bond to precast PDMS track designs incorporating a flow focussing droplet creator, outlined in (fig. 1 (e)(i)). Droplets were subsequently formed on-chip with regular frequency in the range 0.875 – 2.955 Hz, with an average standard deviation of ± 0.06 Hz (50 nl < Volume < 200 nl), and projected through the chip towards the superhydrophobic track. Data showing the probability of a droplet following the superhydrophobic track under the differing flow rates is shown in (fig. 5 (a)). Additionally, as can be seen in the supporting information (ESI5), once droplet production exceeds a certain throughput there is a


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linear drop-off in the percentage of droplets following the desired path. As Metaniemi et al

[4]

demonstrated for unconfined macroscale tracks, the dependant factor in whether or not a droplet will follow the desired trajectory along the patterned track is primarily contingent upon its velocity. In addition to this, it was found that for an enclosed T-junction based system employing a carrier flow that the dependant factor for successful path control was a function of the ratio between the coating fraction CF and the water fraction Wf. This is to say that once the sum total of the droplet areas became larger than the total area of the patterned track the probability of a successful trajectory is reduced significantly. As (fig. 5 (a)) outlines, this drop-off becomes more prominent as the water fraction approaches the coating fraction, where the Coating fraction, CF, can be defined as: / . As these two values get closer together the probability of the droplet traversing the path length can be seen to decrease linearly. Experimentally it was found that the drop-off began to happen when Cf > 0.075. Such a consideration suggests that our system has a limiting value for which repeatability can be ensured, thus limiting the throughput within our current design. Accordingly, for the maximum throughput associated with 100 % completion of the track the maximum droplet velocity was found to be 0.28 (± 0.01) x10-3 ms-1. This demonstrates a reduction in velocity by almost a factor of 10 from the maximum velocity values found by Mertaniemi et al

[4]

for macroscale traversion of patterned unconfined tracks. Thus

demonstrating that the spatial manipulation of droplets within multiphase microfluidic flows is possible but experimentally includes additional considerations to that of the unenclosed macroscale environment. Conclusion Through the careful design and patterning of surfaces with a superhydrophobic polymer coating we show that it is possible to easily provide an alternative, and reliable, toolkit for the manipulation of droplets within microfluidic channels under specific conditions. The porous nature of these substrates opens the door for further targeted applications in the fields of biotechnology and chemical analysis through doping of the substrate for specific chemical interactions. In addition, further enhancement of both the patterning resolution and the superhydrophobic contact angle has been seen to enhance the effects seen throughout this communication and will be investigated in due course.

Acknowledgements This work has been funded in part by a joint ICL and UCL summer placement programme. JBE acknowledges the ERC for a starting investigator grant and EPSRC for funding.



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[1] Y. Zhiqing, C. Hong, Z. Jide, Z. Dejian, L. Yuejun, Z. Xiaoyuan, L. Song, S. Pu, T. Jianxin, C. Xin, Sci. and Technol. Adv. Mater., 2008, 9 (4), 045007. [2] A. Ressine, G. Marko-Varga and T. Laurell, in Biotechnology Annual Review, ed. M. R. El-Gewely, Elsevier, 2007, vol. Volume 13, pp. 149-200. [3] C. R. Crick, I. P. Parkin, Chem. Comunm., 2011, 47, 12059-12061. [4] H. Mertaniemi, R. Forchheimer, O. Ikkala, R. H. A. Ras, Adv. Mater., 2012, 24 (42), 5738-5743. [5] S. Wang, L. Jiang, Adv. Mater., 2007, 19 (21), 3423-3424. [6] H. Mertaniemi, V. Jokinen, L. Sainiemi, S. Franssila, A. Marmur, O. Ikkala, R. H. A. Ras, Adv. Mater., 2011, 23(26), 2911-2914. [7] J. A. Lee, T. J. McCarthy, Macromolecules, 2007, 40 (11), 3965-3969. [8] S. Xing, R. S. Harake, T. Pan, Lab Chip, 2011, 11 (21), 3642-3648. [9] G. Fang, W. Li, X. Wang and G. Qiao, Langmuir, 2008, 24, 11651-11660. [10] K. Joonwon, K. Chang-Jin, "Nanostructured surfaces for dramatic reduction of flow resistance in droplet-based microfluidics", presented at Micro Electro Mechanical Systems, 2002. The Fifteenth IEEE International Conference on, 2002, 2002. [11] M. Joanicot, A. Ajdari, Science, 2005, 309 (5736), 887-888. [12] J. D. Tice, A. D. Lyon, R. F. Ismagilov, Anal. Chim. Acta., 2004, 507 (1), 73-77. [13] A. B. Theberge, F. Courtois, Y. Schaerli, M. Fischlechner, C. Abell, F. Hollfelder, W. T. S. Huck, Angew. Chem. Int. Ed., 2010, 49(34), 5846-5868. [14] X. Niu, S. Gulati, J. B. Edel, A. J. deMello, Lab Chip, 2008, 8, 1837-1841. [15] X. Casadevall i Solvas, M. Srisa-Art, A. J. deMello, J. B. Edel, Anal. Chem., 2010, 82 (9), 3950-3956. [16] F. Gielen, A. J. deMello, T. Cass, J. B. Edel, J. Phys. Chem. B, 2009, 113, 1493-1500. [17] K. Ahn, J. Agresti, H. Chong, M. Marquez, D. A. Weitz, Appl. Phys. Lett., 2006, 88,264105-264103. [18] A. R. Abate, T. Hung, P. Mary, J. J. Agresti and D. A. Weitz, P. Natl. Acad. Sci. USA, 2010, 107, 19163-19166. [19] X. Niu, F. Gielen, A. J. deMello, J. B. Edel, Anal. Chem., 2009, 81, 7321-7325. [20] C. R. Crick, I. P. Parkin, J. of Mater. Chem., 2009, 19, 1074-1076. [21] C. R. Crick, I. P. Parkin, Thin Solid Films, 2010, 518, 4328-4335. [22] S. Sun-Sik, K. Eun-Uk, J. Hee-Soo, K. Ki-Seok, J. Gun-Young, J. Micromech. Microeng., 2009, 19, 105022. [23] M. C. Draper, X. Niu, S. Cho, D. I. James, J. B. Edel, Anal. Chem., 2012, 84 (13), 5801-5808. [24] V. A. Turek, M. P. Cecchini, J. Paget, A. R. Kucernak, A. A. Kornyshev, J. B. Edel, ACS Nano., 2012, 6, 7789-7799. [25] J. Qian, C. K. Law, J. Fluid Mech., 1997, 331, 59-80. [26] M. Srisa-Art, A. J. deMello, J. B. Edel, Anal. Chem., 2007, 79 (17), 6682-6689.



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Figure 1. (a)(i) 5 µl Au NP doped DI water droplet on superhydrophobic polymer substrate, (a)(ii) same 5 µl Au NP doped DI water droplet on an untreated borosilicate glass slide and (b) SEM image of the surface at 10.88 Kx magnification, (c)(i) image of the full velocity control chip with a 4 µm thickness, 400 µm width by 1 cm length SHPS integrated alongside a blown up frame-by-frame representation of the droplet traversing the substrate (c)(ii). (d)(i) Full merging chipset with a SHPS of 4 µ m thickness, ≈200 µ m width and 1 cm length placed within the same channel dimensions as (c), alongside a single frame extract of the merging process in (d)(ii). (e)(i) Full path control chipset alongside a single frame image of droplets following the desired path in (e)(ii). (e) places a 4 µ m thick superhydrophobic track within a wider chamber of width 2.2 mm and height 100 µm.


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Droplet motion over SHS (a)

(i)

(ii)

(iii)

(iv)

(b) Figure 2. (a) Images outlining the various phases in the droplets path over the SHPS. (b) Mean velocity profile for an average of 20 droplets (red circles) transiting across a (i-ii) 4 µm high and (iii-iv) 7 µm high surface. The standard deviation is shown as a black line.



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Figure 3. Mean velocity profile for an average of 20+ droplets (red circles) at a constant (a) total flow rate (Tf) and (b) water fraction (Wf).


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Figure 4. (a) Displays 8 frames elucidating the droplet merging process within continuous channel architecture, (b) Schematic outline of the experimental considerations which are necessary for merging of two droplets at the boundary of a SHPS, (c) A velocity-time profile for the two droplets involved in the merging process.



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Figure 5. (a) Demonstrates the level of path control achievable through application of a patterned SHPS to a microfluidic chamber alongside variation of the applied Wf. Inset (b) displays a single frame taken from the video demonstrating the on-chip control.


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ToC figure


Superhydrophobic Surfaces as an On-Chip ... - ACS Publications

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