Measurement of the total optical angular momentum transfer in optical tweezers

We describe a way to determine the total angular momentum, both spin and orbital, transferred to a particle trapped in optical tweezers. As an example an LG02 mode of a laser beam with varying degrees of circular polarisation is used to trap and rotate an elongated particle with a well defined geometry. The method successfully estimates the total optical torque applied to the particle. For this technique, there is no need to measure the viscous drag on the particle, as it is an optical measurement. Therefore, knowledge of the particle's size and shape, as well as the fluid's viscosity, is not required.Comment: 7 pages, 3 figure

Forces from highly focused laser beams: modeling, measurement and application to refractive index measurements

The optical forces in optical tweezers can be robustly modeled over a broad range of parameters using generalsed Lorenz-Mie theory. We describe the procedure, and show how the combination of experimental measurement of properties of the trap coupled with computational modeling, can allow unknown parameters of the particle - in this case, the refractive index - to be determined.Comment: 5 pages, 4 figures, presented at 17th AIP Congress, Brisbane, 200

Contribuicao Do BPM (Business Process Management) Nas Operacoes Da Cadeia De Suprimentos De Uma Organizacao Sem Fins Lucrativos

Purpose There are many activities that depend on an organization in order to provide conditions for an excellent service. In the present study, there was deepening and exploiting processes and sub processes involving the sector\u27s supply West Paulista Association of the Seventh-day Adventist Church organization, considering the importance of offering supply service to internal customers seeking continuity of work and scope the organization\u27s objectives. The research objective was to present possibilities for improvement in the development of processes and sub processes mentioned by the interviewees\u27 perceptions and their contributions. Method Through data collection through qualitative research aimed to identify the factors and processes directly related to the supply cycle of the organization. Research where stakeholders presented by their perception of the processes and sub processes, as well as suggestions for improvement through the use of tooling BMP (Business Process Management) were used. One semi structured research, where the necessary information to identify existing processes and sub processes were provided, and presented three main processes, purchasing, warehousing and distribution, was developed and; four sub processes, requisitions or purchase orders, budgets, commitments and purchase agreement. Results Associated characteristic aspects and issues were identified that permeate the supplement industry, such as: direct purchases, technology, autonomy, standardizations and specificities. The investigation highlights the possibility of improvements in terms of the characteristics related to the purchasing process, highlighting autonomy, direct purchases, specificities, technology, and regulations. The investigation also revealed that the autonomy process has an intimate relationship with direct purchases. It also highlights the importance of a system to monitor the entire purchase process. Conclusion The final results suggest the possibility of improvement in the organization\u27s culture, assimilating new procedures and standards for operations studied, technology development, restructuring of budgets and approvals processes and the remodeling of the functioning of the supply sector, especially activities related to purchases ; in conducting training, in obtaining and sending feedback to decrease the incidence of rework, correct flaws in communication, limit the exceptions listed and operate with greater predictability. Keywords: supply chain, process, purchasing, warehousing, distribution, sub process, requisition, budget, order, purchase contract, process management, BPM, Model as is model to be

Microrheology using dual beam optical tweezers and ultrasensitive force measurements

We investigate the dynamics of microscopic flow vortices. We create flow vortices by rotation of birefringent particles in optical tweezers. We then use either highly sensitive drag force measurements or video tracking to map the fluid velocity around that particle. We find that we can measure fluid velocities very accurately in the range of 3 to 2,000 mu m/s. The results obtained from these different methods are compared. Velocity profiles obtained for water agree very well with theoretical predictions. In contrast, we find a strong deviation of velocity profiles in a complex fluid from those predicted by simple theory

Formation of an artificial blood vessel: Adhesion force measurements with optical tweezers

We are investigating the formation of a tissue capsule around a foreign body. This tissue capsule can be used as an autologous graft for the replacement of diseased blood vessels or for bypass surgery. The graft is grown in the peritoneal cavity of the recipient and the formation starts with the adhesion of cells to the foreign body. We identify the cell type and measure the adhesion of these cells to foreign materials using optical tweezers. Cell adhesion to macroscopic samples and microspheres is investigated. No difference in the adhesion force was measurable for polyethylene, silicon and Tygon on a scale accessible to optical tweezers. The density of adherent cells was found to vary strongly, being highest on polyethylene. The mean rupture forces for cell-microsphere adhesion ranged from 24 to 39 pN and changed upon preadsorption of bovine serum albumin. For plain microspheres, the highest mean rupture force was found for PMMA, which also showed the highest adhesion probability for the cell-microsphere interaction

Picolitre viscometry using optically rotated particles

Important aspects in the field of microrheology are the studies of the viscosity of fluids within structures with micron dimensions and fluid samples where only microlitre volumes are available. We have quantitatively investigated the performance and accuracy of a microviscometer based on rotating optical tweezers, that requires as little as one microlitre of sample. We have characterised our microviscometer, including effects due to heating, and demonstrated its ability to perform measurements over a large dynamic range of viscosities (at least two orders of magnitude). We have also inserted a probe particle through the membrane of a cell and measured the viscosity of the intramembranous contents. Viscosity measurements of tears have also been made with our microviscometer, which demonstrate its potential use to study un-stimulated eye fluid.Comment: 5 pages, 7 figure

A Constant Torque Micro-Viscometer

We present a technique to measure the viscosity of microscopic volumes of liquid using rotating optical tweezers. The technique can be used when only microlitre (or less) sample volumes are available, for example biological or medical samples, or to make local measurements in complicated micro-structures such as cells. The rotation of the optical tweezers is achieved using the polarisation of the trapping light to rotate a trapped birefringent spherical crystal, called vaterite. Transfer of angular momentum from a circularly polarised beam to the particle causes the rotation. The transmitted light can then be analysed to determine the applied torque to the particle and its rotation rate. The applied torque is determined from the change in the circular polarisation of the beam caused by the vaterite and the rotation rate is used to find the viscous drag on the rotating spherical particle. The viscosity of the surrounding liquid can then be determined. Using this technique we measured the viscosity of liquids at room temperature, which agree well with tabulated values. We also study the local heating effects due to absorption of the trapping laser beam. We report heating of 50-70 K/W in the region of liquid surrounding the particle

Optical microrheology of biopolymers

We use passive and active techniques to study microrheology of a biopolymer solution. The passive technique is video tracking of tracer particles in the biopolymer, a technique which is well established. The active technique is based on rotating optical tweezers, which is used to study viscosity. A method to actively measure viscoelascity using time varying rotation of a particle trapped in optical tweezers is also presented

Microrheology of microlitre samples: Probed with rotating optical tweezers

Microrheology is the study of fluid flows and material deformations on a microscopic scale. The study of viscoelasticity of microscopic structures, such as cells, is one application of microrheometry. Another application is to study biological and medical samples where only a limited volume (microlitres) of fluid is available. This second application is the focus of our work and we present a suitable microrheometer based on optical tweezers. Optical tweezers are an optical trap created by a tightly focused laser beam. The gradient force at this focus acts to trap transparent micron sized particles, which can be manipulated within the surrounding environment. We use the polarisation of the incident field to transfer angular momentum to a trapped spherical birefringent particle. This causes the particle to rotate and measuring the polarisation of the forward scattered light allows the optical torque applied to the sphere to be calculated. From the torque, the viscosity of the surrounding liquid can be found. We present a technique that allows us to perform these measurements on microlitre volumes of fluid. By applying a time-dependent torque to the particle, the frequency response of the liquid an also be determined, which allows viscoelasticity to be measured. This is left as a future direction for this project

Calculation of optical trapping landscapes

Manipulation of micrometer sized particles with optical tweezers can be precisely modeled with electrodynamic theory using Mie's solution for spherical particles or the T-matrix method for more complex objects. We model optical tweezers for a wide range of parameters including size, relative refractive index and objective numerical aperture. We present the resulting landscapes of the trap stiffness and maximum applicable trapping force in the parameter space. These landscapes give a detailed insight into the requirements and possibilities of optical trapping and provide detailed information on trapping of nanometer sized particles or trapping of high index particles like diamond