“…However, since experimental limitations precluded collection of meaningful results when the surfaces were separated by Ͻ0.4 m, this technique could not be used to study ultrathin liquid films. Encouraging results have also been obtained by Helm and co-workers, 7,8 who have measured optical properties of a thin dye layer adsorbed to the mica surfaces of an SFA. They show that the intensity and resonance wavelength transmission peaks from a FabryPerot interferometer contain a wealth of information about the concentration and interactions of confined dye molecules.…”
“…However, since experimental limitations precluded collection of meaningful results when the surfaces were separated by Ͻ0.4 m, this technique could not be used to study ultrathin liquid films. Encouraging results have also been obtained by Helm and co-workers, 7,8 who have measured optical properties of a thin dye layer adsorbed to the mica surfaces of an SFA. They show that the intensity and resonance wavelength transmission peaks from a FabryPerot interferometer contain a wealth of information about the concentration and interactions of confined dye molecules.…”
“…Recently, we started to study dyes in the SFA. There we demonstrated that the shape of the transmission peaks gives additional information about the dye concentration and orientation at the surfaces. , …”
Section: Introductionmentioning
confidence: 76%
“…Now, we take an additional (software) slit, to obtain the wavelength dependent intensity at the center of the fringes (Figure , bottom row). As expected from a cavity, we obtain transmission peaks with Lorentzian shapes , and calculate from the peak position the surface separation. 1 Schematic view of the optical setup.…”
We present a computer-controlled technique to measure the
distance-dependent forces in the surface
forces apparatus. The power of our setup is shown by a measurement
of the repulsive forces between mica
surfaces immersed in a 0.01 M NaCl solution. At close distances we
find an oscillatory force with a
periodicity of 0.30 nm, which roughly corresponds to the diameter of a
water molecule. For the distance
determination we use the standard interferometric technique: the
interferometer consists of a medium
sandwiched between two mica sheets of equal thickness silvered on the
backside. The surface separation
is measured by comparing the resonance wavelength to the one obtained
from contacting mica sheets. By
substitution of the eye by a CCD-camera we improve the reproducibility
of the method and we can determine
the cavity parameters quantitatively.
“…Owing to the technical difficulties of performing the experiment in existing apparatuses, direct measurements of the field-induced orientations of confined molecules have been few. Among the exceptions are measurements of fluorescence depolarization, X-ray diffraction, , and shear-induced optical anisotropy of sheared dye monolayers , within suitably modified surface forces apparatuses. These are special cases, however, and it is clear that for bulk samples there are many additional alternative methods to study molecular orientation.…”
Fluids of mesoscopic thickness can be sheared and their molecular orientation probed concurrently with the new instrument described in this paper. The fluid is confined between parallel optically flat windows whose spacing is controlled, using piezoelectric inchworms, from submicrometer thickness to ∼500 µm, with no essential lower limit apart from surface roughness. Capacitance sensors or optical interferometry is used to monitor spacing between the windows with submicrometer accuracy. Piezoelectric bimorphs are used to apply periodic shear displacements with amplitude 0.l-10 µm and frequency 0.1-700 Hz. Shear-induced molecular alignment during sinusoidal shear cycles is determined, with up to 5 µs time resolution, using step-scan time-resolved infrared spectroscopy. To demonstrate capabilities of this new instrument, we describe an experiment in which shear and electric fields were applied in orthogonal directions to 5-cyanobiphenyl (5CB), a simple nematic liquid crystal. Provided that the molecule lacked the time to relax during the period of oscillation, the molecule tilted back and forth around the equilibrium orientation under the action of small-amplitude oscillating shear. The shear alignment appeared to be proportional to the shear displacement, not to the effective shear rate.
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