Hierarchical micro and nano structured, hydrophilic, superhydrophobic and superoleophobic surfaces incorporated in microfluidics, microarrays and lab on chip microsystems
Post on 24-Mar-2017
Microelectronic Engineering 132 (2015) 135155Contents lists available at ScienceDirect
journal homepage: www.elsevier .com/locate /meeReview ArticleHierarchical micro and nano structured, hydrophilic, superhydrophobicand superoleophobic surfaces incorporated in microfluidics, microarraysand lab on chip microsystemshttp://dx.doi.org/10.1016/j.mee.2014.10.0020167-9317/ 2014 Elsevier B.V. All rights reserved.
Corresponding author.E-mail address: email@example.com (E. Gogolides).Evangelos Gogolides , Kosmas Ellinas, Angeliki TserepiInstitute of Nanoscience and Nanotechnology, NCSR Demokritos, Aghia Paraskevi, Attiki 15310, Greecea r t i c l e i n f o
Article history:Received 31 July 2014Received in revised form 17 September 2014Accepted 5 October 2014Available online 19 October 2014
Keywords:SuperhydrophobicPlasma nanotexturingMicroarraysCell arraysDroplet microfluidicsHydrophilica b s t r a c t
Control of wetting properties at the extremes of wetting states (superhydrophilic and superhydrophobic)is important for many applications, such as self-cleaning, anti-fogging, anti-icing, and antibacterialaction. While significant effort has been devoted to develop and characterize such open surfaces forvarious applications, their incorporation in sensors, microfluidics, and labs on chip, offers new functionaldevices and systems, and poses different requirements compared to open-area surfaces. In this paper,dedicated to the 30 year anniversary of Microelectronic Engineering, we aim to review the extremewetting states of surfaces, their fabrication processes focusing on plasma processing technology, andtheir incorporation into devices and systems. We start with an introduction and terminology forsuperhydrophilic, superhydrophobic, and superoleophobic surfaces, and continue with a review of thefabrication of such surfaces by plasma processing. We then review how such surfaces are incorporatedin microdevices and microsystems, and their applications. We address (a) Hydrophilic capillary pumpsand superhydrophobic valves, (b) Drag reduction in superhydrophobic microchannels and slip lengthincrease, (c) Superhydrophobic surfaces for droplet manipulation, applied to chemical and biologicalanalysis, (d) Biomolecule adsorption control on nanostructured surfaces, and (e) Cell adhesion on suchsurfaces. Finally, we conclude with perspectives and challenges.
2014 Elsevier B.V. All rights reserved.1. Introduction and terminology We start with the definitions of super wetting states for whichControl of wetting properties at the extremes of wetting stateshas acquired an increased interest for many applications, such asself-cleaning, anti-fogging, anti-icing, and antibacterial action.Such surfaces are usually structured at the micro and nano-scaleand possess high or low surface energy. On one extreme lie super-hydrophilic and hydrophilic surfaces, while on the other extremesuperhydrophobic and superoleophobic surfaces are encountered.While significant effort has been devoted to develop and character-ize such open surfaces for various applications, their incorporationin sensors, microfluidics and labs on chip, offers new functionaldevices and systems, and poses different requirements comparedto open-area surfaces. In addition, it narrows the possible fabrica-tion steps, so that compatibility with microsystem fabricationtechnologies is possible.
In this paper, we aim to review the extreme wetting states ofsurfaces, their fabrication processes focusing on plasma processingtechnology, and their incorporation into devices and systems.often multiple terms and some confusion may exist [1,2]. Most ofthe words are combinations of Greek or Latin roots with thefollowing meanings: hydro = water, oleo = elaion = oil, philic =friendly, attracting, phobic = afraid of, repelling, motus = fear,amphi = both (i.e. water and oils for this application), omni = all,everything. We essentially agree with the classification andterminology proposed in , but we prefer to separate oils (typicaledible oils have surface tensions larger than 30 mN/m) from lowersurface tension liquids (e.g. alkanes). We keep the term oleophobicfor oils, and the term omniphobic for all liquids including liquids oflower surface tension compared to oils, considering that omni isa root with stronger meaning encompassing all liquids, whileamphi has a weaker meaning referring both to oils and water.Table 1 below summarizes the surface classifications and theterminology used for such surfaces. The reason for such detailedclassification is that the lower the surface tension the moredifficult it is to obtain a superamphiphobic state. In addition asdiscussed in , there are some counter-intuitive cases where asurface may be oleophobic, but not hydrophobic, hence the needfor the prefix amphi or omni.
Table 1Definitions of extreme wetting states.
State Superhydrophilic Superoleophilic Hydrophilic Oleophilic Hydrophobic Oleophobic Omniphobic Superhydrophobic Superoleophobic Superamphiphobic Superomniphobic
Liquid Water Oils Water Oils Water Oils (c >
c < 30 mN/m
Water Oils Both water and
All, water, oils,
and low surface
(c < 30 mN/m)
150 >150 >150
Hysteresis na na na na >10
Fig. 1. A drop on rough (micro and nanostructured) and smooth surfaces with varying wetting properties: (a) a rough porous superhydrophilic (superoleophilic) surface,(b) a smooth hydrophilic (oleophilic) surface, (c) a smooth hydrophobic (oleophobic) surface, (d) a rough hydrophobic (oleophobic) porous sticky surface where the drop is ina Wenzel sticky state, (e) a hydrophobic SLIPS surface (slippery liquid infused porous surface), (f) a rough superhydrophobic (superoleophobic) surface, where the drop is ina Cassie-Baxter, slippery fakir state.
E. Gogolides et al. /Microelectronic Engineering 132 (2015) 135155 137ate) PET at 10 Pa after 10 min etching introducing the termnanotexture to describe the nanoroughness created, while Fresnaiset al.  fabricated superhydrophobic poly(ethylene) PE in amicrowave plasma of O2 and CF4.
In 2005 Tserepi et al. described a short (13 min) plasma etch-ing step in a high density helicon plasma reactor to create wavystructures of enhanced surface area or high aspect ratio polymericnanofilaments , that could grow several microns in heightwithin a few minutes etching, thus greatly increasing the surfacearea of the polymer. The process used etching gases appropriatefor the polymer being etched (SF6 for Si containing polymers suchas Poly(dimethyl)siloxane PDMS, or O2 for organic polymers suchas Poly(methyl)methacrylate PMMA). This process was alsodescribed as micro and nanotexturing of polymers, and gavesuperhydrophilic nanostructured surfaces, while a subsequentfluorocarbon plasma deposition  could render them superhy-drophobic and antireflective as described in Tserepi et al. ,Vourdas et al.  and Kontziampasis et al. . Plasma nanotex-turing was also used in low density plasma etching reactors bymany other groups, see for example the work by Wohlfart et al.,  for PET, Balu for paper , Fernndez-Blzquez for various organic polymers, Palumbo et al. , Di Mundo et al., Vietro et al. for superhydrophobic polycarbonate forautomotive applications , Tarrade et al. for Poly(ethyleneterephthalate)  and many others.
Fig. 2 shows various micro and nanotextured surfaces createdby plasma etching: Fig. 2(i) shows PET , Fig. 2(ii) showsNanotextured PMMA , Fig. 2(iii) shows PET etched at variouspressures , Fig. 2(iv) shows PC (left) and PS (right) surfaces, Fig. 2(v) shows PDMS surfaces , while Fig. 2(vi) shows fastsuperhydrophobic surface fabrication on organic and inorganicpolymers as evidenced by the abrupt contact angle change versustime of etching for each polymer . The polymeric micro andnanofilaments created by a long plasma nanotexturing processcan be fragile against capillary, bending, or adhesion forces. For thisreason Gnannapa et al. proposed a wetting-drying method tostructurally stabilize them prior to hydrophobization .
While in some cases plasma etching and nanotexturing may bedue to differential etching of components of the material (e.g.crystalline versus amorphous phase), in most cases, nanotexturingis due to the presence of etching inhibitors coming from the reactorwalls or present in the material, and the prevailing anisotropicetching conditions. This has been discussed in detail by Tsougeni for organic polymers and Vlachopoulou et al. [45,46] forPDMS. A detailed study of the role of reactor walls was presentedby Gogolides et al. , while simulation of nanotexture formationand growth was presented by Kokkoris et al. .
Plasma etching and nanotexturing is not only possible withpolymers, but also for silicon and glass . Black Silicon isone such example of random silicon structures which leads tosuperamphiphobicity ,  and overhanging nanostructures. Black Silicon structures can also be transferred to polymersto render them superamphiphobic [52,53].
Plasma depositionwas also used to deposit randomly nanostruc-tured fluorocarbon coatings with superhydrophobic properties (seeFig. 3(i) and (ii)). In addition to the earlyworksmentioned above,wenote also the work of Favia , and Milella for modulated plasmasexploring the mechanisms to deposit ribbon-like or bumpy fluoro-carbon layers , as reviewed also by the same author later on. Vacuum deposition of superhydrophobic coatings was alsoreviewed by Cigala et al. , and Kylian et al. . One of theadvantages of plasma deposition is that it can be used at atmo-spheric pressures aswell. The earlywork of Tsoi et al. whodepositedSiO2 networks  was followed by several more, as for exampleLee et al. who deposited superhydrophobic hydrocarbon polymers, and Wang et al. who used atmospheric plasma jet to depositsuperhydrophobic TiO2 trees (see Fig. 3(ii)); however a longdeposition time was necessary (over 1 h) . Mechanisms ofplasma deposited nanotexture formation and transition from bumpto ribbon structures were discussed by Milella .2.2. Ordered hierarchical, micronanostructured surfaces
It is often required that the micro and nano scale structures areclearly separated, providing a constant, well-defined distancebetween the microscale elements. Such requirements may beposed by the application (e.g. inkjet printer heads) , the fabri-cation method (top down technologies), or by the need for highthermodynamic and mechanical stability offered by suchstructures . Such length scale separation is accomplished bydoing a first lithographic step followed by the etching and thenanotexturing steps. Numerous such examples exist in the litera-ture (see Fig. 4). Marquez-Velasco et al.  created SU8 photore-sist microposts, and they subsequently nanotextured their surfaceusing oxygen plasma (see Fig. 4(i)). Bormashenko et al.  hotembossed PE and then nanotextured it in CF4 plasma to createsuperoleophobic PE surfaces (see Fig. 4(ii)). Cortese et al.  cre-ated micropatterns on PDMS and then nanotextured them in theplasma to create hierarchical superhydrophobic surfaces. Similareffects have appeared for Silicon using Lithography and Deep Si
Fig. 2. (i) Nanotextured PET, 200 W, 10 min Oxygen plasma, Reprinted with permission from Ref.  Copyright 2005 Elsevier. (ii) Nanotextured PMMA in high densityoxygen plasma, by Skarmoutsou et al., Conditions: 1900 W, 100 V bias, 0.75 Pa, 100 sccm O2. Reprinted with permission from Ref.  Copyright 2012 IOP Publishing. (iii)PET surfaces etched at various pressures. Nanotexturing appears at low pressures and is a sign of anisotropic etching mechanisms, reprinted with permission from Ref. Copyright 2010 JohnWiley and Sons. (iv) PC (left) and PS (right) surfaces etched and nanotextured in Oxygen Plasma. Similar structures are obtained in CF4 plasmas, reprintedwith permission from Ref.  Copyright 2012 John Wiley and Sons. (v) PDMS etched in SF6 plasmas, reprinted with permission from Ref.  Copyright 2011 Elsevier. (vi)Fast superhydrophobic surface fabrication on organic and inorganic polymers using oxygen and SF6 plasmas respectively, followed by 30 s fluorocarbon polymer deposition.Contact angle change versus time of etching for each polymer. Reprinted with permission from Ref.  Copyright 2010 Inderscience.
138 E. Gogolides et al. /Microelectronic Engineering 132 (2015) 135155plasma etching. Kwon et al. fabricated micropillars, which weresubsequently nanotextured using XeF2 etching . Nguyen et al.fabricated also micropillars and compared two nanotexturing pro-cesses, metal assisted electroless etching, and chemical vapordeposition of silicon nanowires, the first showing only superhy-drophobicity, the second showing superamphiphobicity  (seeFig. 4(iii)). All such works demonstrate the advantages of hierarchi-cal topography; enhancement of hydrophobicity to superhydrop-hobicity and mostly the significant increase of the stability of theCassie state of such surfaces.
The idea that the slope of the microstructure is important forthe drop suspension was first discussed theoretically by Extrand[68,69]. Tuteja et al. in two successive publications demonstratedthat for superoleophobicity a re-entrant microstructure profile isneeded [70,71], the angle of the profile defining one of their designcriteria. They fabricated micro-hoodoo, mushroom-like Si struc-tures to prove the concept. At the same time, Ahuja et al.  pro-duced Si nanonail superoleophobic surfaces (see Fig. 4(iv)), and byapplying a voltage on suspended drops caused transition from Cas-sie-Baxter to Wenzel regime. Zhao et al. demonstrated that whenSi microstructures are being etched with a Bosch process, the rip-ple itself induces a re-entrant profile leading to superoleophobicity. When an overhang capping layer is added superoleophobic-ity is favored as the thickness of this layer gets smaller (seeFig. 4(v)) . Overhang structures where produced in Silicon bySusarrey et al.  to study drop evaporation, and by Zeniouet al. to study Si nanowire mechanical stability and hydrophobicity(see Fig. 4(vi)) .
Fig. 2 (continued)
E. Gogolides et al. /Microelectronic Engineering 132 (2015) 135155 139Colloidal micronanosphere lithography has early enough beenalso proposed as an alternative cost-effective lithographicmethod . When combined with plasma etching and nanotex-turing, it can lead to nanostructured surfaces spanning the rangefrom superhydroph...