Response Surface Optimisation of Polydimethylsiloxane (PDMS) on Borosilicate Glass and Stainless Steel (SS316) to Increase Hydrophobicity

Particle deposition on the surface of a drying chamber is the main drawback in the spray drying process, reducing product recovery and affecting the quality of the product. In view of this, the potential application of chemical surface modification to produce a hydrophobic surface that reduces the powder adhesion (biofouling) on the wall of the drying chamber is investigated in this study. A hydrophobic polydimethylsiloxane (PDMS) solution was used in the vertical dipping method at room temperature to determine the optimum coating parameters on borosilicate glass and stainless steel substrates, which were used to mimic the wall surface of the drying chamber, to achieve highly hydrophobic surfaces. A single-factor experiment was used to define the range of the PDMS concentration and treatment duration using the Response Surface Methodology (RSM). The Central Composite Rotatable Design (CCRD) was used to study the effects of the concentration of the PDMS solution (X₁, %) and the treatment duration (X₂, h) on the contact angle of the substrates (°), which reflected the hydrophobicity of the surface. A three-dimensional response surface was constructed to examine the influence of the PDMS concentration and treatment duration on contact angle readings, which serve as an indicator of the surface’s hydrophobic characteristics. Based on the optimisation study, the PDMS coating for the borosilicate glass achieved an optimum contact angle of 99.33° through the combination of a PDMS concentration of X₁ = 1% (w/v) and treatment time of X₂ = 4.94 h, while the PDMS coating for the stainless steel substrate achieved an optimum contact angle of 98.31° with a PDMS concentration of X₁ = 1% (w/v) and treatment time of X₂ = 1 h. Additionally, the infrared spectra identified several new peaks that appeared on the PDMS-treated surfaces, which represented the presence of Si-O-Si, Si-CH₃, CH₂, and CH₃ functional groups for the substrates coated with PDMS. Furthermore, the surface morphology analysis using the Field Emission Scanning Electron Microscopy (FESEM) showed the presence of significant roughness and a uniform nanostructure on the surface of the PDMS-treated substrates, which indicates the reduction in wettability and the potential effect of unwanted biofouling on the spray drying chamber. The application of PDMS and PTFE on the optimally coated substrates successfully reduced the amount of full cream milk particles that adhered to the surface. The low surface energy of the treated surface (19–27 mJ/m²) and the slightly higher surface tension of the full cream milk (54–59 mJ/m²) resulted in a high contact angle (102–103°) and reduced the adhesion work on the treated substrates (41–46 mJ/m²) as compared to the native substrates.     

Thermo-responsive polymer brushes as intelligent biointerfaces: preparation via ATRP and characterization

PIPAAm-brush grafted glass substrates with various graft densities and chain lengths were prepared via surface-initiated ATRP. Temperature-dependent physicochemical properties of the surfaces were characterized by means of ATR/FT-IR spectroscopy, XPS, AFM, and contact angle measurements. ATRP conditions influence the amount of grafted PIPAAm and the surface wettability and roughness of the substrate. Fibronectin adsorption and EC adhesion increased with decreasing density of PIPAAm brushes. EC adhesion was diminished with increasing PIPAAm graft length. Thus, the preparation of PIPAAm brush surface with various graft densities and chain lengths using the surface-initiated ATRP is an effective method for modulating thermo-responsive properties of surfaces.     

Binary self-assembled monolayers of alkanethiols on gold: deposition from solution versus microcontact printing and the study of surface nanobubbles

The coadsorption of alkanethiols on noble metals has been recognized for a long time as a suitable means of affording surfaces with systematically varied wettability and other properties. In this article, we report on a comparative study of the composition of the mixed self-assembled monolayers (SAMs) obtained (i) by the coadsorption of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) from ethanol and chloroform onto gold substrates and (ii) by microcontact printing using poly(dimethyl siloxane) (PDMS) stamps. SAMs prepared by coadsorption from solution showed a preferential adsorption of ODT for both solvents, but this trend was reversed in microcontact-printed SAMs when using chloroform as a solvent, as evidenced by contact angle and Fourier transform infrared (FTIR) spectroscopy measurements. An approximately linear relationship between the static contact angle and the degree of swelling with different solvents was observed, which suggests that the surface composition can be controlled by the interaction of the solvent and the PDMS elastomer. The altered preference is attributed to the different partitioning of the two thiols into solvent-swelled PDMS, as shown by (1)H NMR spectroscopy. Finally, molecularly mixed binary SAMs on ODT and MHDA on template-stripped gold were applied to study the effect of surface nanobubbles on wettability by atomic force microscopy (AFM). With a decreasing macroscopic contact angle measured through water, the nanoscopic contact angle was found to decrease as well.     

Controlled adhesion of Salmonella Typhimurium to poly(oligoethylene glycol methacrylate) grafts

Poly(oligoethylene glycol methacrylate), POEGMA, brushes were prepared by surface‐initiated atom transfer radical polymerization (SI‐ATRP) on gold‐coated silicon wafers. Prior to ATRP, the substrates were grafted by brominated aryl initiators via the electrochemical reduction of a noncommercial parent diazonium salt of the formula BF₄^?, ⁺N₂‐C₆H₄‐CH(CH₃)Br. The diazonium‐modified gold plates (Au‐Br) served as macroinitiators for ATRP of OEGMA which resulted in hydrophilic surfaces (Au‐POEGMA) that could be used for two distinct objectives: (i) resistance to fouling by Salmonella Typhimurium; (ii) specific recognition of the same bacteria provided that the POEGMA grafts are activated by anti‐Salmonella. The Au‐POEGMA plates were characterized by XPS, polarization modulation‐infrared reflection‐absorption spectroscopy (PM‐IRRAS) and contact angle measurements. Both Beer‐Lambert equation and Tougaard's QUASES software indicated a POEGMA thickness that exceeds the critical ～10 nm value necessary for obtaining a hydrophilic polymer with effective resistance to cell adhesion. The Au‐POEGMA slides were further activated by trichlorotriazine (TCT) in order to covalently bind anti‐Salmonella antibodies (AS). The antibody‐modified Au‐POEGMA specimens were found to specifically attach Salmonella Typhimurium bacteria. This work is another example of the diazonium salt/ATRP process to provide biomedical polymer surfaces. Copyright © 2011 John Wiley & Sons, Ltd.     

From micro to nano reentrant structures: hysteresis on superomniphobic surfaces

The paper reports on the wetting characterization of two surfaces presenting reentrant shapes at micro- and nanoscale using low surface tension liquids (down to 28 mN/m). On the one hand, mushroom-like microstructures are fabricated by molding poly(dimethylsiloxane) (PDMS) onto a patterned sacrificial photoresist bilayer. On the other hand, zinc oxide nanostructures (ZnO NS) are synthesized by easy and fast chemical bath deposition technique. The PDMS and ZnO NS surfaces are then chemically modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane in vapor phase. Both PDMS and ZnO NS surfaces exhibit a large apparent contact angle (>150°) and contact angle hysteresis varying from 50° to a quasi-null value. This large discrepancy can be ascribed to the length scale and topography of the structures, promoting either a vertical imbibition or a lateral spreading within the roughness.     

Study of hydrophilicity and stability of chemically modified PDMS surface using piranha and KOH solution

Successful realization of various BioMEMS devices demands effective surface modification techniques of PDMS elastomer. This paper presents a detailed report on a simple and cost effective approach for surface modification of PDMS films involving wet chemical treatment in two‐step processes: primarily involving piranha solution followed by KOH dip to improve hydrophilicity and stability of PDMS surface. Chemical composition of the solution and surface treatment condition have been varied and optimized to significantly increase the surface energy. The effect of surface modification of the elastomer after wet chemical treatment is analyzed using contact angle measurement and FTIR‐ATR study. PDMS surface treated in piranha solution with H₂O₂ and H₂SO₄ in the ratio of 2:3 followed by a dip in KOH solution for 15 min duration each, demonstrated a maximum reduction of contact angle to ～27° as compared to untreated sample having a contact angle of ～110°. The removal of hydrophobic methyl group from elastomer surface and subsequent hydrophilization of surface by wet chemical process was confirmed from FTIR‐ATR spectra. This result is also supported by improved adhesion and electrical continuity of deposited aluminum metal film over the modified PDMS surface. Copyright © 2011 John Wiley & Sons, Ltd.     

Bioactive surface modification of mica and poly(dimethylsiloxane) with hydrophobins for protein immobilization

Bioactive surfaces with appropriate hydrophilicity for protein immobilization can be achieved by hydrophobin II (HFBI) self-assembly on mica and polydimethylsiloxane (PDMS) surfaces. X-ray photoelectron spectroscopy and water contact angle measurements illustrated that the surface wettability can be changed from superhydrophobic (PDMS) or superhydrophilic (mica) to moderately hydrophilic, which is suitable for protein (chicken IgG) immobilization on both substrate surfaces. The results suggest that HFBI assembly, one kind of hydrophobin from Trichoderma reesei, may be a versatile and convenient method for the immobilization of biomolecules on diverse substrates, which may have potential applications in biosensors, immunoassays, and microfluidic networks.     

Influence of chemistry and topology effects on superhydrophobic CF(4)-plasma-treated poly(dimethylsiloxane) (PDMS)

Superhydrophobic surfaces are gaining considerable interest in a lot of different applications, and nonetheless, precise control over the wettability properties of such surfaces is still a challenge due to difficulties when controlling the effects independently induced on superhydrophobicity by the chemical and topological surface characteristics. We have fabricated engineered superhydrophobic surfaces onto poly(dimethylsiloxane) (PDMS) substrates by means of suitable CF4-plasma treatments. These treatments allowed the modification of both the morphological properties of the PDMS surface, due to a preferential etching of certain components of its macromolecules, and the chemical ones, by the deposition of a fluorinated layer. Chemical effects were separated from topological ones by performing a double replica molding process of the CF4-plasma-treated surfaces. This allowed us to obtain positive copies of the structured surfaces without the overlaying fluorinated coating affecting the surface chemistry. Such replicated surfaces showed a decrease of the contact angle if compared to the treated ones and therefore evidenced chemistry's weight in superhydrophobicity effects. In particular, we found that, for highly dense columnar-like PDMS microstructures, the effect of the plasma-deposited fluorinated layer covering surfaces produces an enhancement of the contact angle of about 20 degrees .     

Manipulation of Structure on Silicon Surfaces via Chemical Adsorption

Via chemical adsorption, various films were assembled onto silicon surfaces. The structures and properties of the monolayer‐ or bilayer‐modified silicon surfaces, such as Si‐C₁₀H₂₀ CH₂OC(O)CF₃, Si‐C₁₀H₂₀CH₂OH, and Si‐C₁₀H₂₀‐CH₂‐NH‐C₁₈H₃₇, were investigated by various techniques. x‐ray photoelectron spectroscopy (XPS) gave clear proofs of the formation of octadecylamine layer and other kinds of layers on silicon surfaces. The contact angle measurements showed that the wettability of silicon surfaces was dominated by the terminal functional groups of the attached layers. Atomic force microscopy (AFM) observations showed that interesting patterns have formed on the monolayer‐ or bilayer‐modified silicon surfaces. Electrochemical impedance spectra (EIS) measurements showed that the Si‐C₁₀H₂₀CH₂‐NH‐C₁₈H₃₇ has a better ability to prevent charge transfer as compared with that of Si‐C₁₀H₂₀CH₂OH, which may find applications in the area of surface passivations.     

Surface modification of LDPE film by CO2 pulsed laser irradiation

The influence of the pulsed CO₂ laser irradiation on the surface structure of the LDPE film was investigated. Significant changes were observed on the surface of laser treated films as it was verified by the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy and contact angle-measurement. Formation of polar functional groups onto the LDPE surfaces exhibited by the ATR-FTIR spectra was shown to be strongly dependent on the number of the CO₂ laser pulses. The intensity of the polar groups increased with increasing the number of pulses up to two and then slightly decreased at three laser pulses. This was also confirmed with the contact angle measurements in which the sample subjected to two laser pulses showed the highest wettability i.e. the lowest water drop contact angle. The concentration of peroxide groups formed on the surface of the laser treated films was determined quantitatively by UV spectroscopic method using iodide procedure. The latter results showed a similar trend with the results obtained using FTIR spectroscopy.     

Superhydrophobic Polypropylene Surfaces Prepared with TiO2 Nanoparticles Functionalized by Dendritic Polymers

Hybrid inorganic–organic nanomaterials have received increasing interest due to the possibility of implementing different functions and characteristics within a single material. Their functionalities are a consequence of the synergy of the properties of distinct building blocks and are related to their varied natures and spatial locations. In this work, we present the development of superhydrophobic properties on polypropylene (PP) surfaces using hybrid nanomateriales from TiO₂ nanoparticles (NPs) and dendronized polymers. The dendron acryl Behera's amine was successfully grafted on the TiO₂ NP surfaces by Surface‐Initiated Atom Transfer Radical Polymerization (SI‐ATRP) and a core‐brush material was obtained. Finally, PP substrates were coated with NP hybrids to produce superhydrophobic surfaces with water contact angles of over 158 degrees. Controlling the organic silane concentration on the TiO₂ NPs allowed the dendronized process to be driven and thereby permitted the selection of specific wettability properties on PP substrate surfaces with high water adhesion or self‐cleaning conditions. This dendronized effect with consequent steric congestion of the polymeric brushes on the NPs changed their behaviors from Wenzel to the Cassie Baxter state. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2019–2029     

Fabrication of biomimetic superhydrophobic surfaces by a simple flame treatment method

A simple flame treatment method was explored to construct micro/nanostructures on a surface and then fabricate a biomimetic superhydrophobic surface at a relatively low cost. SiO₂‐containing polydimethylsiloxane (PDMS) was used as a substrate. The PDMS replicas with various micropatterned surfaces were fabricated using grass leaf, sand paper, and PET sheet with parallel groove geometry as templates via PDMS replica molding. The PDMS replica surfaces with micron structures and the surface of a flat PDMS sheet as a control sample were further treated by flame. The fabricated surfaces were characterized by scanning electron microscopy and water contact angle measurements. The effect of surface microstructures on the transparency of PDMS was also investigated. The studies indicate that the fine nanoscale structures can be produced on the surfaces of PDMS replicas and a flat PDMS sheet by a flame treatment method, and that the hierarchical surface roughness can be adjusted and controlled by varying the flame treatment time. The flame‐treated surfaces of PDMS replicas and a flat PDMS sheet possess superhydrophobicity and an ultra‐low sliding angle reaching a limiting value of 1°, and the anisotropic wettability of the PDMS replica surface with oriented microgroove structures can be greatly suppressed via flame treatment. The visible light transmittance of the flame‐treated flat PDMS surface decreases with prolonged flame treatment times. Copyright © 2016 John Wiley & Sons, Ltd.     

Biomimetic Fabrication of Polymer Film with High Adhesive Superhydrophobicity by Duplicating Locust Wing Surface

Locust is a common flying insect. Locust wings were used as biomimetic templates to fabricate multi-functional polymer(polydimethylsiloxane, PDMS) films by soft lithography. The microstructure and wettability of the natural and artificial locust wing surfaces were investigated by means of a scanning electron microscope(SEM) and a video-based contact angle meter. The natural locust wing surface exhibits complicated hierarchical structures and high adhesive superhydrophobicity(contact angle 152°). The prepared polymer film faithfully reproduces the surface microstructures of the bio-template, and displays a good hydrophobicity and high adhesion(contact angle 144°). The complex wettability of the natural and artificial locust wing surfaces ascribes to the cooperative effect of hydrophobic composition and multi-dimensional rough microstructures. This work not only promotes our understanding of the wetting mechanism on bio-surfaces, but offers an inexpensive and effective approach for biomimetic fabrication of multi-functional interfacial materials.     

BHK cells behaviour on laser treated polydimethylsiloxane surface

The surface of polydimethylsiloxane (PDMS) was modified using a CO2-pulsed laser to evaluate the changes in physical and biological properties of the treated surface. Attachment of anchorage dependent cells, namely baby hamster kidney (BHK) fibroblastic cells, on PDMS surface was investigated in stationary culture conditions. BHK cell adhesion and growth on the PDMS surfaces were studied using scanning electron microscopy (SEM) and optical microscopy. To evaluate the surface wettability, water drop contact angles were determined. The laser treated PDMS surfaces showed high hydrophobicity and low cell adhesion, no spreading and growth in comparison with the unmodified PDMS. It was found that both the wettability and surface structure of the PDMS surface control cell attachment and growth.     

Plasma and chromic acid treatments of polycarbonate surface to improve coating–substrate adhesion

The influence of Ar/O₂ plasma activation and chromic acid etching of polycarbonate (PC) surface on the adhesion of coating to substrate was systematically studied by cross‐cut and tape peel methods through temperature‐shock aging tests. The differences between the wettabilities and elemental compositions of plasma‐treated and chromic acid‐treated PC surfaces prior to coating deposition were evaluated by contact angle measurements and X‐ray photoelectron spectroscopy. To elucidate the adhesion failure of the coatings, nanoindentation technique was employed for the quantitative assessment of the nanomechanical changes of coating depositions on PCs after temperature‐shock aging tests. The two surface treatments can significantly improve the hydrophilicity and polarity of the PC surface, resulting in excellent adhesion of the coating on the PC substrate. Temperature‐shock aging tests reveal that the adhesion of coating on plasma‐modified substrates is superior to that of chromic acid‐etched substrates. We propose that the improved adhesion of the coating on the plasma‐modified PC can be attributed to the higher wettability and more cross‐linking of C–O–Si bonds at the coating–substrate interface. Copyright © 2013 John Wiley & Sons, Ltd.     

Plasma-induced grafting of polydimethylsiloxane onto polyurethane surface: Characterization and in vitro assay

Plasma-induced grafting of polydimethylsiloxane (PDMS) onto the surface of polyurethane (PU) film. The virgin, plasma treated, and PDMS grafted PU films were characterized by means of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, water drop contact angle measurements, and scanning electron microscopy (SEM). The ATR-FTIR spectrogram of the grafted film showed the new characteristic peaks of PDMS. These grafted surfaces exhibited higher hydrophobicity and homogenous morphology. In vitro cell culture study showed that modified surfaces as well as virgin film were compatible with fibroblast cells. The formation of graft polymers combines the biostability of silicone with excellent physical and mechanical properties of PU.     

Modification of micro-channel filling flow by poly(dimethylsiloxane) surface functionalization with fluorine—Substituted aminonaphthols

Microfluidics based on the capillarity-induced filling of elastomeric channels by a suitable liquid or solution represents a useful route for realizing portable diagnostic devices designed without additional mechanical or electrical micropumps. In this study, an elastomeric mold made of poly(dimethylsiloxane) (PDMS), containing relief patterns placed in intimate contact with a silicon substrate, is utilized to create a continuous network of rectangular micro-channels for the motion of water fluid. The immobilization on activated PDMS surface of suitable functional molecules such as hydrophilic and hydrophobic fluorine-containing aminonaphthols, obtained through a straightforward and versatile synthetic procedure, allowed us to modulate PDMS surface properties depending on the structural characteristics of the employed derivative. In this context, the incorporation of fluorine groups is important for improving biocompatibility of the resulting device, providing surfaces that could be chemically and biologically inert as well as resistant to surface adhesion phenomena. The functionalization from liquid phase of PDMS replicas, involving a covalent derivatization via silanization reaction of the above mentioned compounds to an oxidized PDMS surface, resulted in a successful modification of microfluidic motion of water in rectangular capillaries, moreover contact angle values evidence also how wettability of PDMS films could be modulated, with the fluorinated aminonaphthols fuctionalized PDMS exhibiting higher contact angles.     

The effect of the degree of ionicity of ceramic materials on their wettability by melted sodium chloride

The sessile drop method has been used to study the wettability of hexagonal boron nitride, sapphire, quartz, and polycrystalline silicon carbide by melted sodium chloride in a reducing He–H₂ atmosphere. Melted NaCl completely spreads over sapphire and quartz surfaces and form finite contact angles equal to 51° ± 10° and 77° ± 5° on silicon carbide and hexagonal boron nitride, respectively. The calculated works of salt adhesion to the ceramic substrates increase with the magnitude of the ionic component of ceramic material surface energy.     

Corrosion protection of iron surface modified by poly(methyl methacrylate) using surface-initiated atom transfer radical polymerization (SI-ATRP)

A new N-heterocyclic initiator N-[2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethyl]-2-bromoisobutyramide was synthesized and immobilized on the surface of iron. Methyl methacrylate was grafted from iron substrates via surface-initiated atom transfer radical polymerization (ATRP). The first-order kinetics of poly(methyl methacrylate) (PMMA) grafting from iron revealed the control of ATRP throughout the reaction, and the polymerization reached a high conversion producing polymers with good control of molecular weights (M _n?=?68,800) and low polydispersity indexes (M _w/M _n?<?1.32). The thickness of the polymer brush films was greater than 47 nm after 7 h of reaction time. The grafting density was estimated to be 0.48 chains?nm^?2. The iron surfaces at various stages of modification were characterized by scanning electron microscopy and energy dispersive spectrometer. The analytical results were consistent with a thin compact polymer coating on the surface of iron. Iron surface with grafted PMMA coating showed significant corrosion resistance. This work demonstrated that the surface-initiated ATRP is a versatile means of the surface modification of active metals with well-defined and functionalized polymer brushes.     

Surface properties of hydrosilylated polyolefins annealed in supercritical carbon dioxide

Two hydrosilylated polyolefin compounds are obtained by reacting polypropylene (PP) and polyethylene (PE) with di‐ and multi‐functional hydride‐terminated poly(dimethylsiloxane) (dH‐PDMS and mH‐PDMS), respectively. The PDMS‐rich surface layers on these two samples show different Si concentrations but similar thicknesses. Samples of these materials are annealed in supercritical carbon dioxide (scCO₂) at various temperatures and pressures for different periods of time. On the PP/dH‐PDMS sample, an increase in the annealing temperature does not affect the Si concentration up to 120°C. However, the Si concentration is sharply reduced at T = 150°C at which point the surface appears to be covered by SiO₂ particles. Annealing the PP/dH‐PDMS sample for short times leads to submicron scale SiO₂ particle formation on the surface. The particles form aggregated clusters that spread all over the surface uniformly when the annealing time is extended. However, Si concentration on the PE/mH‐PDMS sample surface is enhanced as the annealing temperature increases, reaching a maximum at an annealing temperature of 100°C. No particle formation is observed on the PE/mH‐PDMS sample surface. The contact angle of both samples is found to increase with annealing temperature. Increasing the scCO₂ pressure leads to a higher Si concentration on the surfaces of both samples. On the other hand, increasing the CO₂ pressure leads to opposite trends in contact angle with the PP/dH‐PDMS sample exhibiting an increasing contact angle and the PE/mH‐PDMS sample exhibiting a decreasing one. Copyright © 2007 John Wiley & Sons, Ltd.