Roughness effects of cellulose and paper substrates on water drop impact and recoil

The impact dynamics of water drops on sized and unsized smooth cellulose films and paper surfaces with controlled roughness levels were studied. The objective was to better understand the effect of roughness on the liquid drop impact dynamics on paper surfaces, isolating from the effect chemical heterogeneity. Drop impact in the first few milliseconds were recorded using high-speed CCD camera and the three-phase contact line movement of the water drop was analyzed. Smooth cellulose film surface and rough paper surface showed similar impact dynamics, suggesting that the surface energy plays a more dominant role than surface roughness. Significantly different dynamic contact angles of water drop on the sized and unsized surfaces were observed during drop impact. The Laplace pressure of the curved spreading front pointing to the centre of a spreading drop on these sized cellulose and paper surfaces reduces the three-phase contact line movement, and leads to smaller maximum spreading diameter. Our results suggest that the water drop spreads on the rough surface is most likely via a “roll-over” action rather than “stick and jump” movements.     

Solid ink-printed filter paper as a green adsorbent material for the solid-phase extraction of UV filters from water samples

This work describes the development of a new green solid-phase extraction approach, which is based on the use of low-cost extraction discs composed of plain filter papers that are covered with a synthetic wax-like coating. The filter papers are printed in a commercial solid ink printer, which dispenses a synthetic wax-like ink on the surface of the paper, to cover the hydrophilic cellulose fibre matrix with an interface of lipophilic domains where non-polar analytes can partition through hydrophobic interactions. The modified paper filters were used to extract hydrophobic organic compounds from water samples following the customary procedure of solid-phase extraction without sorbent preconditioning and needless of high-vacuum sources. As a proof-of-concept application, a series of non-polar organic UV filters were used as model analytes to optimise the extraction parameters and evaluate the performance of the method in spiked water samples. Based on this principle, a new sample preparation platform with low environmental footprint has been developed that enables extraction to be carried out using low-cost, environmental benign and non-toxic conventional materials. The advantages and disadvantages of the method, alongside with its future prospects towards the development of custom-made ‘printed extraction kits’, are envisioned and discussed.     

Solution processed aluminum paper for flexible electronics

As an alternative to vacuum deposition, preparation of highly conductive papers with aluminum (Al) features is successfully achieved by the solution process consisting of Al precursor ink (AlH(3){O(C(4)H(9))(2)}) and low temperature stamping process performed at 110 °C without any serious hydroxylation and oxidation problems. Al features formed on several kinds of paper substrates (calendar, magazine, and inkjet printing paper substrates) are less than ～60 nm thick, and their electrical conductivities were found to be as good as thermally evaporated Al film or even better (≤2 Ω/□). Strong adhesion of Al features to paper substrates and their excellent flexibility are also experimentally confirmed by TEM observation and mechanical tests, such as tape and bending tests. The solution processed Al features on paper substrates show different electrical and mechanical performance depending on the paper type, and inkjet printing paper is found to be the best substrate with high and stable electrical and mechanical properties. The Al conductive papers produced by the solution process may be applicable in disposal paper electronics.     

Stability of monomolecular films of archaebacterial tetraether lipids on silicon wafers: a comparison of physisorbed and chemisorbed monolayers

The monomolecular organisation of symmetric, chemically modified tetraether lipids caldarchaeol-PO(4) was studied using Langmuir film balance, ellipsometry, and atomic force microscopy (AFM). Solid silicon wafer substrates were modified to hydrophobic, hydrophilic, and amino-silanised surfaces; and Langmuir-Blodgett (LB)-films were transferred onto each. LB-caldarchaeol-PO(4) films were subjected to further rinsing with organic solvent and additional physical treatments, to compare their resistance and stability on chemisorbed (amino-silanised) and physisorbed (hydrophobic and hydrophilic) surfaces. The resistance and stability of these monolayer films was characterized by ellipsometry and AFM, and film thickness was determined using ellipsometry. AFM was also employed to observe surface morphology. Monolayer films on hydrophobic surfaces were found to be more resistant to rinsing with organic solvent and additional physical treatments than monolayer films on either amino-silanised or hydrophilic surfaces. The hydrophobic effect with hydrophobic surfaces appears to support the formation of stronger caldarchaeol-PO(4) films on silicon wafer substrates, with increased resistance and stability.     

Kinetics of wetting and spreading by aqueous surfactant solutions

Interest in wetting dynamics processes has immensely increased during the past 10-15 years. In many industrial and medical applications, some strategies to control drop spreading on solid surfaces are being developed. One possibility is that a surfactant, a surface-active polymer, a polyelectrolyte or their mixture are added to a liquid (usually water). The main idea of the paper is to give an overview on some dynamic wetting and spreading phenomena in the presence of surfactants in the case of smooth or porous substrates, which can be either moderately or highly hydrophobic surfaces based on the literature data and the authors own investigations. Instability problems associated with spreading over dry or pre-wetted hydrophilic surfaces as well as over thin aqueous layers are briefly discussed. Toward a better understanding of the superspreading phenomenon, unusual wetting properties of trisiloxanes on hydrophobic surfaces are also discussed.     

An inkjet-printed microfluidic device for liquid-liquid extraction

A microfluidic device for liquid-liquid extraction was quickly produced using an office inkjet printer. An advantage of this method is that normal end users, who are not familiar with microfabrication, can produce their original microfluidic devices by themselves. In this method, the printer draws a line on a hydrophobic and oil repellent surface using hydrophilic ink. This line directs a fluid, such as water or xylene, to form a microchannel along the printed line. Using such channels, liquid-liquid extraction was successfully performed under concurrent and countercurrent flow conditions.     

Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents

Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent. Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results. Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude. W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs. Copyright 1999 Academic Press.     

Spreading of Surfactant Solutions over Hydrophobic Substrates

The spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid-vapor interfacial tension and hydrophilize the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by our experimental observations. Copyright 2000 Academic Press.     

The movement of a water droplet on a gradient surface prepared by photodegradation

A hydrophobic to hydrophilic gradient surface was prepared using the tuned photodegradation of an alkylsilane self-assembled monolayer (SAM) using irradiation of vacuum ultraviolet light (wavelength=172 nm). The water contact angle on the photodegraded SAM surface was adjusted using the intensity and time photoirradiation parameters. The formation of a gradient was confirmed by fluorescent labeling. The water drop moved from the hydrophobic to hydrophilic surface with a velocity that depended on the gradient. The higher the gradient, the faster the water moved. For the first time, we have prepared a gradient surface using photodegradation where the movement of a water drop was regulated by the degree of gradation. Considering that the photodegradation technique can be applied to various surfaces and to lithography, this technique will be useful for various material surfaces.     

Spreading kinetics of water drops on self-assembled monolayers of thiols: significance of inertial effects

Spreading of 5-15 microL water drops on self-assembled monolayers of 1-hexadecanethiol and 11-mercapto-1-undecanol, both homogeneous and mixed compositions, formed on gold-coated silicon wafers or glass slides was recorded with a high-speed video camera. The time (t) evolution of the drop base diameter (D) during spreading was analyzed by a power law-correlation: D approximately t(n). The n value was found to increase from n = 0.3-0.5 for water drops on hydrophobic surfaces characterized by the advancing water contact angle of thetaA = 94-104 degrees to n = 0.5-0.8 on less hydrophobic surfaces (thetaA = 45-66 degrees ). These experimental values were found to be of similar magnitude as the literature values reported for small drops and bubbles, which spread over a variety of different substrates including water and water-ethanol drops on self-assembled monolayers of alkylsilanes, air bubbles in water on glass, molten metals on solid metals and ceramics, hydrocarbon drops on water, and others. Inertial effects, which are often not accounted for in the analysis of spreading results, appear to have an impact on the spreading kinetics of small drops in at least the first few milliseconds of the spreading phenomenon.     

Non-DLVO silica interaction forces in NMP-water mixtures. II. An asymmetric system

The interaction between energetically asymmetric hydrophilic and hydrophobic surfaces has fundamental and practical importance in both industrial and natural colloidal systems. The interaction forces between a hydrophilic silica sphere and a silanated, hydrophobic glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using atomic force microscopy (AFM). A strong and long-range attractive force was observed in pure water and was attributed to the formation of capillary bridges associated with nanoscale bubbles initially present on the hydrophobic surface. When NMP was added, the capillary force and corresponding pull-off force became less attractive, which was explained readily in terms of the surface wettability by the binary solvent mixture. Similar to the case of symmetric (two hydrophilic) surfaces, the range of attraction between the asymmetric surfaces was maximized at around 30 vol % NMP, which is consistent with the formation of a thick adsorbed macrocluster layer on the hydrophilic silica surface.     

Adsorption of amyloid beta-peptide at polymer surfaces: a neutron reflectivity study.

The adsorption of amyloid beta-peptide at hydrophilic and hydrophobic modified silicon-liquid interfaces was characterized by neutron reflectometry. Distinct polymeric films were used to obtain noncharged (Formvar), negatively (sodium poly(styrene sulfonate)) and positively charged (poly(allylamine hydrochloride)) hydrophilic as well as hydrophobic surfaces (polystyrene and a polysiloxane-dodecanoic acid complex). Amyloid beta-peptide was found to adsorb at positively charged hydrophilic and hydrophobic surfaces, whereas no adsorbed layer was detected on hydrophilic noncharged and negatively charged films. The peptide adsorbed at the positively charged film as patches, which were dispersed on the surface, whereas a uniform layer was observed at hydrophobic surfaces. The thickness of the adsorbed peptide layer was estimated to be approximately 20 A. The peptide formed a tightly packed layer, which did not contain water. These studies provide information about the affinity of the amyloid beta-peptide to different substrates in aqueous solution and suggest that the amyloid fibril formation may be driven by interactions with surfaces.     

Friction and nanowear of polystyrene against hydrophobic and hydrophilic substrates

The ambition of this study is to analyze the role of interfacial interactions in friction and nanowear of polystyrene, by comparing friction against hydrophobic wafers (methyl‐terminated) and hydrophilic wafers (hydroxyl‐terminated) as a function of sliding velocity and normal force. Friction experiments are performed with a translation tribometer and nanowear investigation is achieved by using atomic force microscopy (AFM) analysis of the wafer surfaces after friction. Experimental results show that the friction coefficients measured on hydrophilic surfaces are always larger than those obtained with hydrophobic surfaces, indicating a relationship between friction and interfacial interactions. Elsewhere, AFM analysis shows that polystyrene transfer appears for a higher normal force in the case of hydrophobic substrates compared to hydrophilic one. However, the corresponding tangential (or friction) force necessary to detect transfer is quite similar for both types of substrates, indicating that the initial wear of polystyrene occurs for a similar threshold interfacial shear. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2449–2454, 2006     

Laser-treated hydrophobic paper: an inexpensive microfluidic platform

We report a method for fabricating inexpensive microfluidic platforms on paper using laser treatment. Any paper with a hydrophobic surface coating (e.g., parchment paper, wax paper, palette paper) can be used for this purpose. We were able to selectively modify the surface structure and property (hydrophobic to hydrophilic) of several such papers using a CO(2) laser. We created patterns down to a minimum feature size of 62±1 μm. The modified surface exhibited a highly porous structure which helped to trap/localize chemical and biological aqueous reagents for analysis. The treated surfaces were stable over time and were used to self-assemble arrays of aqueous droplets. Furthermore, we selectively deposited silica microparticles on patterned areas to allow lateral diffusion from one end of a channel to the other. Finally, we demonstrated the applicability of this platform to perform chemical reactions using luminol-based hemoglobin detection.     

A comparison of spreading behaviors of Silwet L-77 on dry and wet lotus leaves

Trisiloxane surfactants are widely used in pesticide applications as adjuvants to promote spray drop spreading on leaves. The efficacy of the spray is related to the wetting of plant surfaces. The surface (composite or wetted) formed by the liquid drop instantly contacting with the substrate is vital to the spreading. In this paper the spreading behaviors of surfactant solutions on dry and previous wet lotus leaf surfaces were studied. It was found that the drop spreading on the wet surface was obviously easier than on the dry surface, which was rational to the existence of water in the grooves of the wet surface. The spreading of Silwet L-77 aqueous drops on the wet lotus leaf surface is mainly controlled by the surface tension gradient along the air-liquid interface.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

XPS investigations of ink‐jet printed paper

Different ink‐jet printed paper materials were investigated using X‐ray photoelectron spectroscopy (XPS) yielding the elemental composition of the near‐surface region of the papers. We found significant differences with respect to the detected elements and their atomic concentrations in the different inks studied here. Two different groups of inks could be identified by means of a lower ratio of the O and C atomic concentrations and lower concentrations in specific trace elements like Mg, Na and Si. High‐resolution spectra of C 1s and O 1s core levels allowed a detailed determination of the chemical state of the respective elements. On the basis of a detailed deconvolution of these XPS signals, significant differences between all the investigated ink‐jet printed papers were found, thereby allowing their discrimination. The applicability of the measurements and, more generally, the XPS technique for forensic investigations of paper are discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Incorporation into paper of cellulose triacetate films containing semiconductor nanoparticles

CdSe/ZnS quantum dots (QDs) were embedded in films of cellulose triacetate (CTA) to give clear films with the broad absorbance and well-defined, size-tunable fluorescence characteristic of QDs. The relative quantum yields of the QDs in polymer were compared to that of the initial QDs dispersed in toluene. Alkaline hydrolysis of the film surfaces to regenerated cellulose rendered the previously hydrophobic CTA film surfaces hydrophilic and compatible with aqueous papermaking. Films containing combinations of different sized QDs gave more complex emission patterns. Small pieces of fluorescent films were added to pulp slurries and incorporated into laboratory paper sheets through hydrogen bonding between the regenerated cellulose film surfaces and cellulosic pulp fibers. The film system (cellulose ester bulk/cellulose surface) can be used to incorporate hydrophobic particles or molecules compatible with solutions of cellulosic polymers into paper products at both high and low loadings. QDs in paper may prove useful for security applications, such as sheets with unique optical signatures.     

Probing proteins on functionalized silicon surfaces using matrix-assisted laser desorption/ionization mass spectrometry

Flat H-terminated Si(111) substrates modified with alkyl monolayers terminated with hydrophobic and hydrophilic functional groups were prepared using known surface functionalization methods and characterized by FTIR, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The surfaces were then used for the study of non-specific binding of proteins from complex mixtures (using standard mixture of proteins with average molecular weight approximately 6-66 kDa) by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Protein adsorption on these surfaces (following on-probe fractionation of the mixture) was found to be dependent on the nature of surface functional groups, and nature and pH of rinsing solutions used. The results obtained in this work demonstrate that simple silicon-based surface modifications can be effective for direct analysis of complex mixtures by MALDI-MS. Preliminary results obtained using similarly functionalized porous silicon substrates proved that such substrates are (due to their increased surface areas) better performing than flat silicon.     

Cleaning and hydrophilization of atomic force microscopy silicon probes

The silicon surface of commercial atomic force microscopy (AFM) probes loses its hydrophilicity by adsorption of airborne and package-released hydrophobic organic contaminants. Cleaning of the probes by acid piranha solution or discharge plasma removes the contaminants and renders very hydrophilic probe surfaces. Time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy investigations showed that the native silicon oxide films on the AFM probe surfaces are completely covered by organic contaminants for the as-received AFM probes, while the cleaning methods effectively remove much of the hydrocarbons and silicon oils to reveal the underlying oxidized silicon of the probes. Cleaning procedures drastically affect the results of adhesive force measurements in water and air. Thus, cleaning of silicon surfaces of the AFM probe and sample cancelled the adhesive force in deionized water. The significant adhesive force values observed before cleaning can be attributed to formation of a bridge of hydrophobic material at the AFM tip-sample contact in water. On the other hand, cleaning of the AFM tip and sample surfaces results in a significant increase of the adhesive force in air. The presence of water soluble contaminants at the tip-sample contact lowers the capillary pressure in the water bridge formed by capillary condensation at the AFM tip-sample contact, and this consequently lowers the adhesive force.