722 research outputs found
Proximity effect on hydrodynamic interaction between a sphere and a plane measured by Force Feedback Microscopy at different frequencies
In this article, we measure the viscous damping and the associated
stiffness of a liquid flow in sphere-plane geometry in a large frequency
range. In this regime, the lubrication approximation is expected to dominate.
We first measure the static force applied to the tip. This is made possible
thanks to a force feedback method. Adding a sub-nanometer oscillation of the
tip, we obtain the dynamic part of the interaction with solely the knowledge of
the lever properties in the experimental context using a linear transformation
of the amplitude and phase change. Using a Force Feedback Microscope (FFM)we
are then able to measure simultaneously the static force, the stiffness and the
dissipative part of the interaction in a broad frequency range using a single
AFM probe. Similar measurements have been performed by the Surface Force
Apparatus with a probe radius hundred times bigger. In this context the FFM can
be called nano-SFA
Imaging material properties of biological samples with a Force Feedback Microscope
Mechanical properties of biological samples have been imaged with a
\textit{Force Feedback Microscope}. Force, force gradient and dissipation are
measured simultaneously and quantitatively, merely knowing the AFM cantilever
spring constant. Our first results demonstrate that this robust method provides
quantitative high resolution force measurements of the interaction The little
oscillation imposed to the cantilever and the small value of its stiffness
result in a vibrational energy much smaller than the thermal energy, reducing
the interaction with the sample to a minimum. We show that the observed
mechanical properties of the sample depend on the force applied by the tip and
consequently on the sample indentation.
Moreover, the frequency of the excitation imposed to the cantilever can be
chosen arbitrarily, opening the way to frequency-dependent studies in
biomechanics, sort of spectroscopic AFM investigations
Spectroscopic investigation of local mechanical impedance of living cells
The mechanical properties of PC12 living cells have been studied at the
nanoscale with a Force Feedback Microscope using two experimental approaches.
Firstly, the local mechanical impedance of the cell membrane has been mapped
simultaneously to the cell morphology at constant force. As the force of the
interaction is gradually increased, we observed the appearance of the
sub-membrane cytoskeleton. We shall compare the results obtained with this
method with the measurement of other existing techniques. Secondly, a
spectroscopic investigation has been performed varying the indentation of the
tip in the cell membrane and consequently the force applied on it. In contrast
with conventional dynamic atomic force microscopy techniques, here the small
oscillation amplitude of the tip is not necessarily imposed at the cantilever
first eigenmode. This allows the user to arbitrarily choose the excitation
frequency in developing spectroscopic AFM techniques. The mechanical response
of the PC12 cell membrane is found to be frequency dependent in the 1 kHz - 10
kHz range. The damping coefficient is reproducibly observed to decrease when
the excitation frequency is increased.Comment: 8 pages, 8 figure
Out of equilibrium anomalous elastic response of a water nano-meniscus
We report the observation of a transition in the dynamical properties of
water nano-menicus which dramatically change when probed at different time
scales. Using a AFM mode that we name Force Feedback Microscopy, we observe
this change in the simultaneous measurements, at different frequencies, of the
stiffness G'(N/m), the dissipative coefficient G''(kg/sec) together with the
static force. At low frequency we observe a negative stiffness as expected for
capillary forces. As the measuring time approaches the microsecond, the dynamic
response exhibits a transition toward a very large positive stiffness. When
evaporation and condensation gradually lose efficiency, the contact line
progressively becomes immobile. This transition is essentially controlled by
variations of Laplace pressure
Variation of Burkholderia cenocepacia cell wall morphology and mechanical properties during cystic fibrosis lung infection, assessed by atomic force microscopy
The influence that Burkholderia cenocepacia adaptive evolution during long-term infection in cystic fibrosis (CF) patients has on cell wall morphology and mechanical properties is poorly understood despite their crucial role in cell physiology, persistent infection and pathogenesis. Cell wall morphology and physical properties of three B. cenocepacia isolates collected from a CF patient over a period of 3.5 years were compared using atomic force microscopy (AFM). These serial clonal variants include the first isolate retrieved from the patient and two late isolates obtained after three years of infection and before the patient's death with cepacia syndrome. A consistent and progressive decrease of cell height and a cell shape evolution during infection, from the typical rods to morphology closer to cocci, were observed. The images of cells grown in biofilms showed an identical cell size reduction pattern. Additionally, the apparent elasticity modulus significantly decreases from the early isolate to the last clonal variant retrieved from the patient but the intermediary highly antibiotic resistant clonal isolate showed the highest elasticity values. Concerning the adhesion of bacteria surface to the AFM tip, the first isolate was found to adhere better than the late isolates whose lipopolysaccharide (LPS) structure loss the O-antigen (OAg) during CF infection. The OAg is known to influence Gram-negative bacteria adhesion and be an important factor in B. cenocepacia adaptation to chronic infection. Results reinforce the concept of the occurrence of phenotypic heterogeneity and adaptive evolution, also at the level of cell size, form, envelope topography and physical properties during long-term infection
Blobs in Wolf-Rayet Winds: Random Photometric and Polarimetric Variability
Some isolated Wolf-Rayet stars present random variability in their optical
flux and polarization. We make the assumption that such variability is caused
by the presence of regions of enhanced density, i.e. blobs, in their envelopes.
In order to find the physical characteristics of such regions we have modeled
the stellar emission using a Monte Carlo code to treat the radiative transfer
in an inhomogeneous electron scattering envelope. We are able to treat multiple
scattering in the regions of enhanced density as well as in the envelope
itself. The finite sizes of the source and structures in the wind are also
taken into account. Most of the results presented here are based on a parameter
study of models with a single blob. The effects due to multiple blobs in the
envelope are considered to a more limited extent. Our simulations indicate that
the density enhancements must have a large geometric cross section in order to
produce the observed photopolarimetric variability. The sizes must be of the
order of one stellar radius and the blobs must be located near the base of the
envelope. These sizes are the same inferred from the widths of the sub-peaks in
optical emission lines of Wolf-Rayet stars. Other early-type stars show random
polarimetric fluctuations with characteristics similar to those observed in
Wolf-Rayet stars, which may also be interpreted in terms of a clumpy wind.
Although the origin of such structures is still unclear, the same mechanism may
be working in different types of hot stars envelopes to produce such
inhomogeneities.Comment: Accepted to ApJ. 17 pages + 6 figure
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