Biomedical subjects

N B Viana

Publications and source records attributed to N B Viana.

4 recordsLinked to original sources

PubMed2006

Characterization of objective transmittance for optical tweezers.

We have measured the overall transmittance of a laser beam through an oil immersion objective as a function of the transverse size of the laser beam, using the dual-objective method. Our results show that the objective transmittance is not uniform and that its dependence on the radial beam's position can be modeled by a Gaussian function. This property affects the intensity distribution pattern in the sample region and should be taken into account in theoretical descriptions of optical tweezers. Moreover, one must consider this position dependence to determine the local laser power delivered at the sample region by the dual-objective method, especially when the beam overfills the objective's back entrance. If the transmittance is assumed to be uniform, the local power is overestimated.

Computer Simulation↗

PubMed2004

DNA-psoralen interaction: a single molecule experiment.

By attaching one end of a single lambda-DNA molecule to a microscope coverslip and the other end to a polystyrene microsphere trapped by an optical tweezers, we can study the entropic elasticity of the lambda-DNA by measuring force versus extension as we stretch the molecule. This powerful method permits single molecule studies. We are particularly interested in the effects of the photosensitive drug psoralen on the elasticity of the DNA molecule. We have illuminated the sample with different light sources, studying how the different wavelengths affect the psoralen-DNA linkage. To do this, we measure the persistence length of individual DNA-psoralen complexes.

Chemistry, Physical↗

PubMed2002

Dynamic light scattering from an optically trapped microsphere.

Using a single microscope objective lens to optically trap, illuminate, and collect backscattered light of a dielectric microsphere, we measure the temporal-intensity-autocorrelation functions (ACFs), and intensity profiles to obtain the trap stiffness and friction coefficient of the bead. This is an interesting study of an harmonically bound Brownian particle, with nanometer resolution. We extend the work of Bar-Ziv et al. [Phys. Rev. Lett. 78, 154 (1997)] to more general situations allowing for the use of our simpler geometry in other applications. As examples, we present measurements of the parallel Stokes friction coefficient on the trapped bead as a function of its distance from a surface and the entropic force of a single lambda-DNA molecule.

Bacteriophage lambda↗

PubMed1999

Experimental test of the Warren-Langer model in nematic-isotropic planar interfaces.

In a directional solidification apparatus, the recoil of the nonsteady planar nematic-isotropic interface of the liquid crystal 8CB doped with hexachloroethane was measured, for different pulling velocities. Results agree very well with the predictions of our two-sided extension of Warren and Langer's one-sided model [Phys. Rev. E 47, 2702 (1993)], therefore supporting the validity of their ansatz about the evolution of the dopant concentration field. From the comparison between experiment and theory we obtain values for the segregation and diffusion coefficients of hexachloroethane in 8CB comparable to those found in the literature and measured by other methods. Using the same procedure, we measured the value of the segregation coefficient of 8CB doped with water as a function of applied sinusoidal electric field perpendicular to the sample, along the homeotropic direction. The segregation coefficient increases with electric field. In addition, preliminary results on the cellular instability in this system show that the capillary length of the pattern also increases with electric field. To our knowledge, this is the first binary system with continuously tunable segregation coefficient and capillary length.

Journal Article↗