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[en] Atomic force microscopy (AFM) has been used to measure the adhesion force between a flat Si(0 0 1) wafer and a micrometer sized flat silicon AFM tip. Force–distance curves have been recorded at different setpoints in order to elucidate their individual effect on the derived adhesion force. No dependence of the derived adhesion force on the applied load has been detected, making sure that no plastic changes in the morphology of either tip and/or sample occur. Other setpoints as the residence time of the tip at the substrate, the relative humidity, the size of the tip and the retraction velocity of the tip have been varied systematically. We have found that the adhesion force depends strongly on the velocity of the z-piezo and the tip size while, at least within the 0.5–41 s time window, the residence time does not have any measurable effect on the adhesion force. The time scale of the retraction varies between 0.2 and 25 s. The increase of the adhesion force with increasing retraction speed is ascribed to the viscous force. Finally, the adhesion force increases with increasing relative humidity.
[en] The adhesion force between a surface and the tip of an atomic force microscope cantilever has been determined by recording force-distance curves with an atomic force microscope. Flat tips with a diameter of 2 μm were used to mimic the adhesion between two parallel surfaces. In such a configuration, the location for the formation and breaking of the capillary water neck is a stochastic by nature, significantly different from that of a spherical tip. The adhesion force is measured as a function of relative humidity for smooth and chemically etched Si(1 0 0) surfaces. The roughness of the etched substrate reduces the adhesion by more than an order of magnitude, depending on the exact value of the relative humidity. The adhesion force increases with increasing humidity until a relative humidity of about 70%. Beyond a relative humidity of 70% a decrease of the adhesion force is observed. We anticipate that the latter is due to a decrease of the cross section of the water neck at the snap off point with increasing relative humidity.
[en] Thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) was grafted onto the cotton fabric by atom transfer radical polymerization (ATRP). Introducing 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) onto the surface, the density of PNIPAAm chains can be adjusted because of the competitive reactions of (3-aminopropyl) triethoxysilane (APS) and PFDTS. With the appropriate ratio of APS and PFDTS, the cotton fabric can be switched from superhydrophilic to superhydrophobic by controlling temperature. The prepared cotton fabric may find application in functional textiles, soft and folding superhydrophobic materials.
[en] Functionally nanostructured fibres were prepared by silver sputter coating at room temperatures. Atomic force microscopy (AFM) and environmental scanning electron microscopy (ESEM) were employed to study the topography and wetting behaviour of the fibres, respectively. The AFM results indicate a significant difference in the morphology before and after the sputter coating. The observations by AFM also show the change in the morphology of the fibres with coating time. The ESEM examination of wetting was achieved by changing the relative humidity in the ESEM chamber. The relative humidity was raised up to 100% by controlling the specimen's temperature and the chamber pressure to produce water condensation on the fibre surfaces. The ESEM observation also indicates the change in the wetting properties of the fibres
[en] Highlights: • The adhesion force at ∼15% humidity increases logarithmically with contact time. • The contact time plays a dominant role among the experimental parameters that have an influence on the adhesion force. • The adhesion forces with different normal loads and piezo velocities can be quantitatively obtained just by figuring out the length of contact time. • The adhesion force with repeated contacts at one location is in accordance with the contact time dependence. - Abstract: The influences of contact time, normal load, piezo velocity, and measurement number of times on the adhesion force between two silicon surfaces were studied with an atomic force microscope (AFM) at low humidity (17–15%). Results show that the adhesion force is time-dependent and increases logarithmically with contact time until saturation is reached, which is related with the growing size of a water bridge between them. The contact time plays a dominant role among these parameters. The adhesion forces with different normal loads and piezo velocities can be quantitatively obtained just by figuring out the length of contact time, provided that the contact time dependence is known. The time-dependent adhesion force with repeated contacts at one location usually increases first sharply and then slowly with measurement number of times until saturation is reached, which is in accordance with the contact time dependence. The behavior of the adhesion force with repeated contacts can be adjusted by the lengths of contact time and non-contact time. These results may help facilitate the anti-adhesion design of silicon-based microscale systems working under low humidity.
[en] Highlights: ► The effect of the short-term UV radiation on properties of PPTA fibers was studied. ► The properties include surface properties, mechanical and thermal resistance. ► Surfaces of radiated fibers are rougher and have more oxygen-based functional groups. ► Radiation does not change the chemistry and crystalline structure of fiber core. ► The structure-property relation of PPTA fibers during the UV radiation was build. - Abstract: The influence of the short-term (<20 h) ultraviolet (UV) radiation (at 60 ± 3 °C with a relative humidity of 50 ± 1 RH%) on the integrated performance of poly(p-phenylene terephthalaramide) fibers was comprehensively studied, and the mechanism behind the influence was intensively discussed by detecting the overall changes in both chemical and morphological structures. Results demonstrate that the short-term UV radiation has different effect on the core part from the surface part of PPTA fibers. Specifically, the short-term UV radiation slightly decreases the crystalline index without changing the chemical structure of the core part of PPTA fibers; while that introduces a large amount of oxygen atoms on the surfaces of UV-KF fibers, and induces a distinctive increase in the surface roughness of fibers even the irradiation time is only 1 h. These structural changes make UV-KF fibers show decreased contact angle and improved wettability while remaining the outstanding glass transition temperature. All parameters of tensile properties including tenacity, break extension, energy to break and modulus almost linearly decrease as the irradiation time extends; however, these parameters almost level off when the irradiation time is longer than 3 h. After irradiated for 18 h, the tenacity, break extension, energy to break of UV-KF fiber is 88%, 90%, and 86% of the corresponding value of original fiber, respectively.
[en] Highlights: • We measured the adhesion between a flat AFM tip and a rough Si wafer. • The adhesion does not depend on contact time or contact pressure and is virtually independent of tip size. • The adhesion increases with relative humidity. • The adhesion strongly decreases with increasing retraction speed. • The velocity dependence is indicative of predominant capillary forces. - Abstract: We have used atomic force microscopy (AFM) to measure the snap-off forces between a micron sized flat silicon AFM tip and a rough Si(0 0 1) surface. The current paper is a natural continuation of our previous paper (Çolak et al., 2014), dealing with snap-off forces between an identical flat tip and flat Si(0 0 1). Within the applied experimental parameter windows we observed no dependence of the snap-off forces on the applied normal loads (3–18 μN) and residence times (0.5–35 s) for the current flat-on-rough geometry as was the case for the former flat-on-flat geometry. The snap-off forces were found to increase with relative humidity in both geometries. As in the case of the flat-on-flat contact geometry, a strong dependence of the snap-off forces on the retraction speed of the tip was observed. Here, we find a strong decrease of the snap-off forces with increasing tip speed, especially at low velocities in the range 40–1000 nm/s for the flat-on-rough geometry. This is in contrast with the flat-on-flat geometry, where we found a strong increase of the snap-off force with increasing tip speed. These observations are explained in terms of a cross-over of the importance of capillary forces and viscous forces. We suggest that the relative importance of both forces can be checked via variation of the tip speed.
[en] Highlights: • The humidity dramatically affects the contrast of scanning polarization force microscopy (SPFM) imaging on mica surface. • This influence roots in the sensitive dielectric constant of mica surface to the humidity change. • A strategy of controllable and repeatable imaging the local dielectric properties of nanomaterials with SPFM is proposed. - Abstract: Scanning polarization force microscopy (SPFM) is a useful surface characterization technique to visually characterize and distinguish nanomaterial with different local dielectric properties at nanometer scale. In this paper, taking the individual one-atom-thick graphene oxide (GO) and reduced graphene oxide (rGO) sheets on mica as examples, we described the influences of environmental humidity on SPFM imaging. We found that the apparent heights (AHs) or contrast of SPFM imaging was influenced significantly by relative humidity (RH) at a response time of a few seconds. And this influence rooted in the sensitive dielectric constant of mica surface to the RH change. While dielectric properties of GO and rGO sheets were almost immune to the humidity change. In addition, we gave the method to determine the critical humidity at which the contrast conversion happened under different conditions. And this is important to the contrast control and repeatable imaging of SPFM through RH adjusting. These findings suggest a strategy of controllable and repeatable imaging the local dielectric properties of nanomaterials with SPFM, which is critically important for further distinguishment, manipulation, electronic applications, etc.
[en] Highlights: • Zinc oxide (ZnO) targets have been surface treated using a frequency doubled nanosecond pulsed Nd:YAG laser at laser fluence levels ranging between 2 and 9 J/cm2. • Our observations establish that laser irradiation provides an effective technique for generation of surface macro porosity in case of ZnO pellets. • Extent of surface porosity and the mean pore size could be controlled by appropriately varying the incident laser fluence. • Such laser treated ZnO surfaces with enhanced surface porosity and large size pores (mean pore area ∼2–5 μm2) can serve as potential candidate for humidity sensors with high sensitivity and fast response time, particularly in high humidity range. - Abstract: Surface porosity and pore size distribution of sensing material greatly influence performance parameters such as sensitivity, reproducibility and response time of sensors. Various approaches have been employed to generate surface porosity having varying pore size distribution. This paper presents our results on pulsed laser irradiation based surface microstructuring of sintered zinc oxide (ZnO) pellets leading to generation of enhanced surface porosity. ZnO targets have been surface treated using a frequency doubled nanosecond pulsed Nd:YAG laser at laser fluence levels ranging between 2 and 9 J/cm2. Our observations establish that laser irradiation provides an effective technique for generation of surface macro porosity in case of ZnO pellets. Also, extent of surface porosity and the mean pore size could be controlled by appropriately varying the incident laser fluence. Such laser treated ZnO surfaces with enhanced surface porosity and large size pores can serve as potential candidate for humidity sensors with high sensitivity and fast response time, particularly in high humidity range
[en] An original approach based on X-ray photoelectron spectroscopy (XPS) is proposed to study the influence of the surrounding humidity on the hydrolysis-condensation reactions of five titanium alkoxides in thin films. More precisely, the influence of the nature of the ligands (propoxide, butoxide, isopropoxide, phenoxide, and 9H-carbazole-9-yl-ethyl-oxy) on the reaction rate was evidenced. The reaction advancement was evaluated by comparing XPS chemical compositions to theoretical compositions calculated for all the possible rates. XPS chemical environment information allowed validating the reliability of this approach through the evaluation of the condensation state. In both approaches, the influence of the steric hindrance on the reactivity of titanium alkoxides was highlighted to be similar to what has been previously observed in solution. Theses results corroborate the validity of our XPS approach to determine titanium alkoxide hydrolysis-condensation reactions in the specific application of thin films