15 research outputs found
Double-layer force suppression between charged microspheres
In this paper we propose a protocol to suppress double-layer forces between two microspheres immersed in a dielectric medium, being one microsphere metallic at a controlled potential
ψ
M
and the other a charged one either metallic or dielectric. The approach is valid for a wide range of distances between them. We show that, for a given distance between the two microspheres, the double-layer force can be totally suppressed by simply tuning
ψ
M
up to values dictated by the linearized Poisson-Boltzmann equation. Our key finding is that such values can be substantially different from the ones predicted by the commonly used proximity force approximation, also known as the Derjaguin approximation, even in situations where the latter is expected to be accurate. The proposed procedure can be used to suppress the double-layer interaction in force spectroscopy experiments, thus paving the way for measurements of other surface interactions, such as Casimir dispersion forces
Probing the screening of the Casimir interaction with optical tweezers
We measure the colloidal interaction between two silica microspheres in
aqueous solution in the distance range from m to m with the
help of optical tweezers. When employing a sample with a low salt
concentration, the resulting interaction is dominated by the repulsive
double-layer interaction which is fully characterized. The double-layer
interaction is suppressed when adding M of salt to our sample, thus
leading to a purely attractive Casimir signal. When analyzing the experimental
data for the potential energy and force, we find good agreement with
theoretical results based on the scattering approach. At the distance range
probed experimentally, the interaction arises mainly from the unscreened
transverse magnetic contribution in the zero-frequency limit, with nonzero
Matsubara frequencies providing a negligible contribution. In contrast, such
unscreened contribution is not included by the standard theoretical model of
the Casimir interaction in electrolyte solutions, in which the zero-frequency
term is treated separately as an electrostatic fluctuational effect. As a
consequence, the resulting attraction is too weak in this standard model, by
approximately one order of magnitude, to explain the experimental data.
Overall, our experimental results shed light on the nature of the thermal
zero-frequency contribution and indicate that the Casimir attraction across
polar liquids has a longer range than previously predicted.Comment: 19 pages, 9 figures; updated references; added a detailed discussion
of the subtraction procedure leading to the interaction potentia
Nonadditivity of critical Casimir forces
In soft condensed matter physics, effective interactions often emerge due to the spatial confinement of fluctuating fields. For instance, microscopic particles dissolved in a binary liquid mixture are subject to critical Casimir forces whenever their surfaces confine the thermal fluctuations of the order parameter of the solvent close to its critical demixing point. These forces are theoretically predicted to be nonadditive on the scale set by the bulk correlation length of the fluctuations. Here we provide direct experimental evidence of this fact by reporting the measurement of the associated many-body forces. We consider three colloidal particles in optical traps and observe that the critical Casimir force exerted on one of them by the other two differs from the sum of the forces they exert separately. This three-body effect depends sensitively on the distance from the critical point and on the chemical functionalisation of the colloid surfaces
Probing the Casimir force with optical tweezers
We propose to use optical tweezers to probe the Casimir interaction between microspheres inside a liquid medium for geometric aspect ratios far beyond the validity of the widely employed proximity force approximation. This setup has the potential for revealing unprecedented features associated to the non-trivial role of the spherical curvatures. For a proof of concept, we measure femtonewton double-layer forces between polystyrene microspheres at distances above 400 nm by employing very soft optical tweezers, with stiffness of the order of fractions of a fN/nm. As a future application, we propose to tune the Casimir interaction between a metallic and a polystyrene microsphere in saline solution from attraction to repulsion by varying the salt concentration. With those materials, the screened Casimir interaction may have a larger magnitude than the unscreened one. This line of investigation has the potential for bringing together different fields including classical and quantum optics, statistical physics and colloid science, while paving the way for novel quantitative applications of optical tweezers in cell and molecular biology
Probing the Casimir force with optical tweezers
We propose to use optical tweezers to probe the Casimir interaction between
microspheres inside a liquid medium for geometric aspect ratios far beyond the
validity of the widely employed proximity force approximation. This setup has
the potential for revealing unprecedented features associated to the
non-trivial role of the spherical curvatures. For a proof of concept, we
measure femtonewton double layer forces between polystyrene microspheres at
distances above nm by employing very soft optical tweezers, with
stiffness of the order of fractions of a fN/nm. As a future application, we
propose to tune the Casimir interaction between a metallic and a polystyrene
microsphere in saline solution from attraction to repulsion by varying the salt
concentration. With those materials, the screened Casimir interaction may have
a larger magnitude than the unscreened one. This line of investigation has the
potential for bringing together different fields including classical and
quantum optics, statistical physics and colloid science, while paving the way
for novel quantitative applications of optical tweezers in cell and molecular
biology.Comment: 7 pages, 5 figure
Cytotoxic triterpenoid saponins from the roots of Cephalaria gigantea
Three new oleanane-type saponins, giganteosides L (1)(I), M (2) and N (3) along with eight known ones were isolated from the roots of Cephalaria gigantea. Their structures were established as 3-O-[-D-galactopyranosyl-(12)--D-glucuronopyranosyl]-28-O-[-D-glucopyranosyl-(16)--D-glucopyranosyl]-oleanolic acid, 3-O-[-D-galactopyranosyl-(12)--D-glucuronopyranosyl]-28-O-[-D-glucopyranosyl-(16)--D-glucopyranosyl]-hederagenin, 3-O-[-L-rhamnopyranosyl-(12)--D-glucuronopyranosyl]-28-O-[-D-glucopyranosyl-(16)--D-glucopyranosyl]-hederagenin, resp., by means of spectroscopic methods (1D and 2D NMR, HR-ESI-MS). Cytotoxic activity of monodesmosides was investigated in vitro using three cancer cell lines, namely, human non pigmented melanoma MEL-5 and human leukemia HL-60. Giganteosides D (4) and E (5) showed antiproliferative effect on human cell lines with IC50 values in the range 3.15-7.5 M