Stiffer optical tweezers through real-time feedback control

Using real-time re-programmable signal processing we connect acousto-optic steering and back-focal-plane interferometric position detection in optical tweezers to create a fast feedback controlled instrument. When trapping 3 µm latex beads in water we find that proportional-gain position-clamping increases the effective lateral trap stiffness ~13-fold. A theoretical power spectrum for bead fluctuations during position-clamped trapping is derived and agrees with the experimental data. The loop delay, ~19 µs in our experiment, limits the maximum achievable effective trap stiffness

A Scalar Wigner Theory for Polarized Light in Nonlinear Kerr Media

A scalar Wigner distribution function for describing polarized light is proposed in analogy with the treatment of spin variables in quantum kinetic theory. The formalism is applied to the propagation of circularly polarized light in nonlinear Kerr media and an extended phase space evolution equation is derived along with invariant quantities. We further consider modulation instability as well as the extension to partially coherent fields.Comment: 6 page

Seasonal and Taxonomic Differences in the Size and Activity of the Thyroid Glands in Birds

Author Institution: University of Illinois, Champaign, and Metropolitan Park District, Cleveland, OhioThe thyroid glands of the house sparrow and other small birds in the Cleveland region have greater secretory activity during late autumn and winter than during late spring and summer. Evidence for this is the presence, during the winter, of high epithelial cells surrounding the follicles, of smaller follicles, and of lesser volumes and weights of the whole thyroid. Large-sized species have larger thyroids, both absolute and relative to body weight, than do small species. With certain precautions, changes in the size of the thyroids serve as a useful index of inverse variations in secretory activity intra-specifically, but not inter-specifically

Conformation of a Polyelectrolyte Complexed to a Like-Charged Colloid

We report results from a molecular dynamics (MD) simulation on the conformations of a long flexible polyelectrolyte complexed to a charged sphere, \textit{both negatively charged}, in the presence of neutralizing counterions in the strong Coulomb coupling regime. The structure of this complex is very sensitive to the charge density of the polyelectrolyte. For a fully charged polyelectrolyte the polymer forms a dense two-dimensional "disk", whereas for a partially charged polyelectrolyte the monomers are spread over the colloidal surface. A mechanism involving the \textit{overcharging} of the polyelectrolyte by counterions is proposed to explain the observed conformations.Comment: 4 pages, 4 figures (6 EPS files

Interaction between clients and physiotherapists in group exercise classes in geriatric rehabilitation

The aim of this paper is to explore how older people construct their interaction in group exercise classes in geriatric rehabilitation and what is their contribution to the interaction. Discourse analysis was employed and data, consisting of seven videotaped group-based exercise sessions, were collected from 52 older people (aged 66–93 years) and nine rehabilitation professionals in seven rehabilitation centres. Four discourse categories were found. In “taciturn exercising”, older people remained verbally silent but physically active. In “submissive disagreeing”, older people opposed the professionals’ agenda by displaying reluctant consent to proposals. In “resilient endeavouring”, older adults persisted on their course of action, regardless of the disapproval of the professionals. In “lay helping”, older people initiated spontaneous encouragement, but also gave verbal and physical assistance to their peers. Older people's meaningful contribution to interaction, whilst it may challenge the institutional flow of activities, can constitute an integral part of the re-ablement process of rehabilitation

Broadband, unpolarized repumping and clearout light sources for Sr+^+ single-ion clocks

Future transportable optical clocks require compact and reliable light sources. Here, broadband, unpolarized repumper and state clearout sources for Sr+ single-ion optical clocks are reported. These turn-key devices require no frequency stabilization nor external modulators. They are fiber based, inexpensive, and compact. Key characteristics for clock operation are presented, including optical spectra, induced light shifts and required extinction ratios. Tests with an operating single-ion standard show a clearout efficiency of 100%. Compared to a laser-based repumper, the achievable fluorescence rates for ion detection are a few tens of per cent lower. The resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the ion system. Similar repumper light sources could be made for Ca+ (866 nm) and Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure

Force generation in small ensembles of Brownian motors

The motility of certain gram-negative bacteria is mediated by retraction of type IV pili surface filaments, which are essential for infectivity. The retraction is powered by a strong molecular motor protein, PilT, producing very high forces that can exceed 150 pN. The molecular details of the motor mechanism are still largely unknown, while other features have been identified, such as the ring-shaped protein structure of the PilT motor. The surprisingly high forces generated by the PilT system motivate a model investigation of the generation of large forces in molecular motors. We propose a simple model, involving a small ensemble of motor subunits interacting through the deformations on a circular backbone with finite stiffness. The model describes the motor subunits in terms of diffusing particles in an asymmetric, time-dependent binding potential (flashing ratchet potential), roughly corresponding to the ATP hydrolysis cycle. We compute force-velocity relations in a subset of the parameter space and explore how the maximum force (stall force) is determined by stiffness, binding strength, ensemble size, and degree of asymmetry. We identify two qualitatively different regimes of operation depending on the relation between ensemble size and asymmetry. In the transition between these two regimes, the stall force depends nonlinearly on the number of motor subunits. Compared to its constituents without interactions, we find higher efficiency and qualitatively different force-velocity relations. The model captures several of the qualitative features obtained in experiments on pilus retraction forces, such as roughly constant velocity at low applied forces and insensitivity in the stall force to changes in the ATP concentration.Comment: RevTex 9 pages, 4 figures. Revised version, new subsections in Sec. III, removed typo

Preliminary design study of a baseline MIUS

Results of a conceptual design study to establish a baseline design for a modular integrated utility system (MIUS) are presented. The system concept developed a basis for evaluating possible projects to demonstrate an MIUS. For the baseline study, climate conditions for the Washington, D.C., area were used. The baseline design is for a high density apartment complex of 496 dwelling units with a planned full occupancy of approximately 1200 residents. Environmental considerations and regulations for the MIUS installation are discussed. Detailed cost data for the baseline MIUS are given together with those for design and operating variations under climate conditions typified by Las Vegas, Nevada, Houston, Texas, and Minneapolis, Minnesota. In addition, results of an investigation of size variation effects, for 300 and 1000 unit apartment complexes, are presented. Only conceptual aspects of the design are discussed. Results regarding energy savings and costs are intended only as trend information and for use in relative comparisons. Alternate heating, ventilation, and air conditioning concepts are considered in the appendix

Gravitational solution to the Pioneer 10/11 anomaly

A fully relativistic modified gravitational theory including a fifth force skew symmetric field is fitted to the Pioneer 10/11 anomalous acceleration. The theory allows for a variation with distance scales of the gravitational constant G, the fifth force skew symmetric field coupling strength omega and the mass of the skew symmetric field mu=1/lambda. A fit to the available anomalous acceleration data for the Pioneer 10/11 spacecraft is obtained for a phenomenological representation of the "running" constants and values of the associated parameters are shown to exist that are consistent with fifth force experimental bounds. The fit to the acceleration data is consistent with all current satellite, laser ranging and observations for the inner planets.Comment: 14 pages, 3 figures, 3 tables. typo's were corrected at Equations (4) and (12) and a third table including our predictions for the anomalous perihelion advance of the planets was adde