217 research outputs found
Thermochemical scanning probe lithography of protein gradients at the nanoscale
Patterning nanoscale protein gradients is crucial for studying a variety of cellular processes in vitro. Despite the recent development in nano-fabrication technology, combining nanometric resolution and fine control of protein concentrations is still an open challenge. Here, we demonstrate the use of thermochemical scanning probe lithography (tc-SPL) for defining micro- and nano-sized patterns with precisely controlled protein concentration. First, tc-SPL is performed by scanning a heatable atomic force microscopy tip on a polymeric substrate, for locally exposing reactive amino groups on the surface, then the substrate is functionalized with streptavidin and laminin proteins. We show, by fluorescence microscopy on the patterned gradients, that it is possible to precisely tune the concentration of the immobilized proteins by varying the patterning parameters during tc-SPL. This paves the way to the use of tc-SPL for defining protein gradients at the nanoscale, to be used as chemical cues e.g. for studying and regulating cellular processes in vitro
The interplay between apparent viscosity and wettability in nanoconfined water
Understanding and manipulating fluids at the nanoscale is a matter of growing scientific and technological interest. Here we show that the viscous shear forces in nanoconfined water can be orders of magnitudes larger than in bulk water if the confining surfaces are hydrophilic, whereas they greatly decrease when the surfaces are increasingly hydrophobic. This decrease of viscous forces is quantitatively explained with a simple model that includes the slip velocity at the water surface interface. The same effect is observed in the energy dissipated by a tip vibrating in water perpendicularly to a surface. Comparison of the experimental data with the model shows that interfacial viscous forces and compressive dissipation in nanoconfined water can decrease up to two orders of magnitude due to slippage. These results offer a new understanding of interfacial fluids, which can be used to control flow at the nanoscale
Kinetics of capillary condensation in nanoscopic sliding friction
The velocity and humidity dependence of nanoscopic sliding friction has been studied on CrN and diamondlike carbon surfaces with an atomic force microscope. The surface wettability is found to be decisive. Partially hydrophilic surfaces show a logarithmic decrease of friction with increasing velocity, the slope of which varies drastically with humidity, whereas on partially hydrophobic surfaces we confirm the formerly reported logarithmic increase. A model for the thermally activated nucleation of water bridges between tip and sample asperities fully reproduces the experimental data
The 2/3 power law dependence of capillary force on normal load in nanoscopic friction
During the sliding of an atomic force microscope (AFM) tip on a rough hydrophilic surface, water capillary bridges form between the tip and the asperities of the sample surface. These water bridges give rise to capillary and friction forces. We show that the capillary force increases with the normal load following a 2/3 power law. We trace back this behavior to the load induced change of the tip-surface contact area which determines the number of asperities where the bridges can form. An analytical relationship is derived which fully explains the observed interplay between humidity, velocity, and normal load in nanoscopic friction
Nanotribology of carbon based thin films: the influence of film structure and surface morphology
The tribological behavior of carbon based thin films is strongly influenced by their chemical composition, polycrystalline structure and surface morphology. We present friction measurements on laser deposited amorphous carbon and carbon nitride (CNchi) thin films using atomic force microscopy. We studied the friction behavior of these films in relation with their structure and surface morphology resulting from the applied deposition parameters. We found high nanoscopic friction for amorphous carbon thin films, medium friction for CN chi and very low friction for graphite. Finally we discuss our findings in terms of the microscopic mechanisms of energy dissipation underlying the observed friction behavior. (C) 2001 Elsevier Science B.V. All rights reserved
Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon
This paper examines friction as a function of the sliding velocity and
applied normal load from air to UHV in a scanning force microscope (SFM)
experiment in which a sharp silicon tip slides against a flat Si(100) sample.
Under ambient conditions, both surfaces are covered by a native oxide, which is
hydrophilic. During pump-down in the vacuum chamber housing the SFM, the
behavior of friction as a function of the applied normal load and the sliding
velocity undergoes a change. By analyzing these changes it is possible to
identify three distinct friction regimes with corresponding contact properties:
(a) friction dominated by the additional normal forces induced by capillarity
due to the presence of thick water films, (b) higher drag force from ordering
effects present in thin water layers and (c) low friction due to direct
solid-solid contact for the sample with the counterbody. Depending on
environmental conditions and the applied normal load, all three mechanisms may
be present at one time. Their individual contributions can be identified by
investigating the dependence of friction on the applied normal load as well as
on the sliding velocity in different pressure regimes, thus providing
information about nanoscale friction mechanisms
Slow dynamics and aging of a confined granular flow
We present experimental results on slow flow properties of a granular
assembly confined in a vertical column and driven upwards at a constant
velocity V. For monodisperse assemblies this study evidences at low velocities
() a stiffening behaviour i.e. the stress necessary to obtain
a steady sate velocity increases roughly logarithmically with velocity. On the
other hand, at very low driving velocity (), we evidence a
discontinuous and hysteretic transition to a stick-slip regime characterized by
a strong divergence of the maximal blockage force when the velocity goes to
zero. We show that all this phenomenology is strongly influenced by surrounding
humidity. We also present a tentative to establish a link between the granular
rheology and the solid friction forces between the wall and the grains. We base
our discussions on a simple theoretical model and independent grain/wall
tribology measurements. We also use finite elements numerical simulations to
confront experimental results to isotropic elasticity. A second system made of
polydisperse assemblies of glass beads is investigated. We emphasize the onset
of a new dynamical behavior, i.e. the large distribution of blockage forces
evidenced in the stick-slip regime
Film structure of epitaxial graphene oxide on SiC: Insight on the relationship between interlayer spacing, water content, and intralayer structure
Chemical oxidation of multilayer graphene grown on silicon carbide yields
films exhibiting reproducible characteristics, lateral uniformity, smoothness
over large areas, and manageable chemical complexity, thereby opening
opportunities to accelerate both fundamental understanding and technological
applications of this form of graphene oxide films. Here, we investigate the
vertical inter-layer structure of these ultra-thin oxide films. X-ray
diffraction, atomic force microscopy, and IR experiments show that the
multilayer films exhibit excellent inter-layer registry, little amount (<10%)
of intercalated water, and unexpectedly large interlayer separations of about
9.35 {\AA}. Density functional theory calculations show that the apparent
contradiction of "little water but large interlayer spacing in the graphene
oxide films" can be explained by considering a multilayer film formed by carbon
layers presenting, at the nanoscale, a non-homogenous oxidation, where
non-oxidized and highly oxidized nano-domains coexist and where a few water
molecules trapped between oxidized regions of the stacked layers are sufficient
to account for the observed large inter-layer separations. This work sheds
light on both the vertical and intra-layer structure of graphene oxide films
grown on silicon carbide, and more in general, it provides novel insight on the
relationship between inter-layer spacing, water content, and structure of
graphene/graphite oxide materials.Comment: 23 pages, 4 figure
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