We present an optical realization of a thermal ratchet. Directed motion of Brownian particles in water is induced by modulating in time a spatially periodic but asymmetric optical potential. The net drift shows a maximum as a function of the modulation period. The experimental results agree with a simple theoretical model based on diffusion.PACS numbers: 05.40.+j Let us consider a Brownian particle diffusing in a one-dimensional periodic well-shaped potential.If the potential height is much larger than the thermal noise, the particle is localized in a minimum. Suppose that this potential is asymmetric and characterized by two length scales Af and Ab (forward and backward) and assume that Ab is larger than Af (time r = 0 in Fig. 1). In an equilibrium situation, not net motion of particles can be induced by a periodic potential, since there is no large scale gradients. However, a time modulation of such a potential, when asymmetric, can induce motion in the following way: Turn the potential off; the particle diffuses freely (time r~r, « in Fig. 1). We call Pf the probability that the particle diffuses forward by more than Af during the time r"«(and similarly Pb for the backward probability).Switching the potential on again after a time~, ff forces the particle to the forward well with a probability Pf and to the backward one with a probability Pq (time r = r,ff in Fig. 1). We define as J = Pf -Pb, the probability current for a particle to advance one step in the periodic potential. Because Ab is larger than Af, Pb is smaller than Pf and the drift is nonzero. As proposed earlier, the time modulation of a periodic asymmetric potential creates directed motion of thermally fluctuating particles [1]. Similar models of engines that extract work from random noise have been recently proposed under the denomination of "thermal ratchets" [2 -6]. These models may have some connection with biological motor proteins [7 -14].How does one experimentally realize such a spatially periodic but asymmetric forcing of Brownian particles?One way is to deposit two metallic films on a glass substrate in a periodic but asymmetric fashion, so that applying an ac electric field through these electrodes creates the desired potential for colloidal particles in an aqueous solution.Recent experiments using such a setup confirmed the induced drift [15,16]. However, hydrodynamic interactions and the complicated electrical response of charges in water limited these experiments to only qualitative agreement with theory. In this Letter, to avoid hydrodynamic interactions we study only one particle (a 1.5 p, m diameter polystyrene 7='Toff Pb (1-Pb-Pf) Pf FIG. 1. The asymmetric potential is drawn as the thickline. The forward and backward length scales defining the asymmetry are Af and Ab. The particle probability densities are drawn as thin lines. At time 7-= 0, the particle is localized and the probability density is sharply peaked. For times 7.~~, «, the potential is off and the particle diffuses freely. At time T 7 ff the potential is back on and t...

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Rheological experiments were carried out on aqueous dispersions of cetyltrimethylammoniumhydroxynaphthalenecarboxylate (CTAHNC) as a function of temperature. The results indicate the formation of very long elongated wormlike micelles at temperatures higher than about 50 °C, conferring to the system a very high viscosity. This behavior is explained by the combined effect of a large end cap energy and a low ionization degree resulting from a strong binding of the weakly soluble counterions. At lower temperature the surfactant forms a much more fluid vesicle phase, which is observed by videomicroscopy. Experiments performed on mixtures of CTAHNC and of cetyltrimethylammonium bromide (CTAB) show also a vesicle to micelle transition for a ratio of CTAB/CTAHNC that decreases upon increasing the temperature. The rheological behavior of the micellar phase obtained by mixing CTAB and CTAHNC is similar to that obtained for other charged micellar solutions.

Most genetic regulatory mechanisms involve protein-DNA interactions. In these processes, the classical Watson-Crick DNA structure sometimes is distorted severely, which in turn enables the precise recognition of the specific sites by the protein. Despite its key importance, very little is known about such deformation processes. To address this general question, we have studied a model system, namely, RecA binding to double-stranded DNA. Results from micromanipulation experiments indicate that RecA binds strongly to stretched DNA; based on this observation, we propose that spontaneous thermal stretching f luctuations may play a role in the binding of RecA to DNA. This has fundamental implications for the protein-DNA binding mechanism, which must therefore rely in part on a combination of f lexibility and thermal f luctuations of the DNA structure. We also show that this mechanism is sequence sensitive. Theoretical simulations support this interpretation of our experimental results, and it is argued that this is of broad relevance to DNA-protein interactions.

Two-photon laser scanning microscopy (2PLSM) is an important tool for in vivo tissue imaging with sub-cellular resolution, but the penetration depth of current systems is potentially limited by sample-induced optical aberrations. To quantify these, we measured the refractive index n' in the somatosensory cortex of 7 rats in vivo using defocus optimization in full-field optical coherence tomography (ff-OCT). We found n' to be independent of imaging depth or rat age. From these measurements, we calculated that two-photon imaging beyond 200 µm into the cortex is limited by spherical aberration, indicating that adaptive optics will improve imaging depth.

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High resolution optical microscopy is essential in neuroscience but suffers from scattering in biological tissues. It therefore grants access to superficial layers only. Recently developed techniques use scattered photons for imaging by exploiting angular correlations in transmitted light and could potentially increase imaging depths. But those correlations ('angular memory effect') are of very short range and, in theory, only present behind and not inside scattering media. From measurements on neural tissues and complementary simulations, we find that strong forward scattering in biological tissues can enhance the memory effect range (and thus the possible field-of-view) by more than an order of magnitude compared to isotropic scattering for ∼1 mm thick tissue layers.

We create tailored microstructures, consisting of complexes of lipid membranes with self-assembled biopolymer shells, to study the fundamental properties and interactions of these basic components of living cells. We measure the mechanical response of these artificial structures at the micrometer scale, using optical tweezers and single-particle tracking. These systems exhibit rich dynamics that illustrate the viscoelastic character of the quasi-two-dimensional biopolymer network. We present a theoretical model relating the rheological properties of these membranes to the observed dynamics.