****NON-TECHNICAL ABSTRACT**** How do molecular motors coordinate their movements to power our muscles? By what principles do such nanometer-scale machines exploit molecular chaos to do their work? Answers to these questions would not only provide valuable insights into natural systems and processes, but also could provide a basis for developing new technology. This research program seeks these answers through experimental studies on model systems that participate in the same physical processes, but whose motivating forces can be tuned and whose behavior lends itself to detailed analysis. These systems are created from very small microscopic particles (colloidal particles) moving through force fields exerted by computer-designed holograms. The colloidal particles? three-dimensional motions are measured with very high resolution using holographic video microscopy and analyzed using the latest developments in the theory of nonequilibrium statistical physics. Recently, this approach has revealed the existence of Brownian vortexes, noise-driven micromachines that do work even in quiescent force fields. Insights into activation and synchronization of natural and artificial micromachines should emerge from a systematic study of Brownian vortexes created from colloidal spheres and specifically crafted beams of light. The combination of holographic experimental techniques used in this project defines the state of the art in this field. In addition to powering this research program, holographic control over the microscopic world will continue to take center stage in New York University's Scientific Frontiers program, a K-12 educational outreach activity that brings hundreds of New York City schoolchildren to NYU for hands-on laboratory experiences.
This experimental program combines holographic optical trapping with holographic video microscopy to probe the statistical physics of individual and interacting colloidal particles moving through non-conservative force fields. Recent developments of optical micromanipulation techniques have created new opportunities to exert precisely controlled forces on microscopically textured systems. Phase gradients in holographically projected traps can give rise to forces and torques that generically violate conservation of mechanical energy. Colloidal particles in phase-enabled optical traps therefore constitute an exceptionally flexible test-bed for new ideas in nonequilibrium statistical physics. The experimental program will exploit new methods of holographic video microscopy to measure the three-dimensional trajectories of optically trapped colloidal particles with nanometer resolution. The holographic optical traps will be designed to optimize both conservative and non-conservative forces. The resulting rich data sets will provide direct insights into the single-particle dynamics, inter-particle interactions, and many-body collective behavior of driven dissipative steady states. Each element of this project, from holographic control of light-induced forces, to synchronization and entropy production in coupled arrays of stochastic heat engines involves the resolution of outstanding scientific and technological questions. The techniques developed for this program, furthermore, will continue to take center stage in New York University's Scientific Frontiers program, a K-12 educational outreach activity that brings hundreds of New York City schoolchildren to NYU for hands-on laboratory tours.
Optical knots, tractor beams, and a motor that runs backward as it warms up The research program supported by Grant Number DMR-0855741 uses the forces and torques exerted by computer-designed hologramsto organize micrometer-scale objects into three-dimensional functional systems, and to learn from their behavior how many-body systems in general organize themselves under the influence of viscous damping, external forcing, and thermal noise. The principles learned from these studies apply to self-organizing systems in general, including many aspects of biology, and can be applied to developing new technologies that assemble and repair themselves. Novel experimental techniques developed for these studies, including versatile new approaches to optical micromanipulation and powerful analytical tools based on holographic video microscopy, already have been commercialized and are being applied in areas as diverse as medical diagnostics and industrial process control and quality assurance. The optical micromanipulation techniques at the heart of these studies are based on the holographic optical trapping technique originally invented by the Principal Investigator's group in 1997. Under support of the present program, the Principal Investigator's group developed a comprehensive theory for the forces exerted by light on microscopic objects, and used that theory to design beams of light that exertprecisely specified forces over precisely designated paths in threedimensions. Using this technique, the group designed and demonstrated the world's first practical tractor beams: beam of light that have a remarkable ability to transport illuminated objects upstream to their source. Long a staple of science fiction, tractor beams based onthe solenoidal modes created for these studies now are being readiedfor real-life space missions. The same technique used to create solenoidal tractor beams alsosuccessfully projects beams of light whose lines of force form closedknots. This is the first experimental demonstration by any techniqueof a knotted force field made to order. Knot-carryingbeams of light show promise for plasmaphysics, where they could help to overcome the instabilities thathave plagued such applications as fusion power generation. In addition to creating novel force fields with light, this research program has pioneered exciting new approaches to monitoring their influence based on holographic video microscopy. This technique uses a conventional video camera to record holograms of microscopic systems. The Principal Investigator's group has developed new ways to analyze each holographic snapshot in a video stream to extract the three-dimensional position of each object in the sample with nanometer resolution, while simultaneously monitoring its size, shape and refractive index with part-per-thousand resolution. No other technique provides so much particle-resolved information about colloidal dispersions. In addition to meeting the present project's needs to track colloidal particles as they move through optical force fields, holographic tracking and characterization are being adopted rapidly by industrial partners who need to characterize their particle-based products. The combination of optical micromanipulation and holographic tracking also is useful for measuring the interactions between colloidal particles, and between particles and surfaces, with unprecedented speed accuracy and resolution. This capability also has immediate applications, most notably in medical diagnostics. Combining these technologies has made possible the discovery of a previously unrecognized class of motor whose archetype is a spherical bead jiggling around in water within a trap created from focused light. Like a motor, this machine transforms energy from a reservoir into motion and heat. The energy in this case comes from the light itself. Unlike a regular motor, however, this machine also requires heat in the form of random thermal forces to work. Each kick the bead receives from fluctuations in the water drives it away from its stationary point at the bottom of the optical trap and out into a wind of non-conservative forces in the light field beyond. These optical forces bias the sphere's fluctuations as it finds its way back into its trap. The resulting circulating motion has been termed a Brownian vortex. Since their discovery in our optical trapping experiments, analogous Brownian vortexes have been recognized in several biological and biochemical systems, and also in financial networks. One surprising feature of a Brownian vortex is that its circulation can change direction with changes in temperature. Understanding how a Brownian vortex chooses its path through the force field is one of the outstanding questions emerging from this work. The combination of holographic control and holographic characterization pioneered by the present program opens up a new horizon for research in soft-matter physics and biophysics. Substantial further work remains in capitalizing on this project's initial advances in volumetric control of light-matter interactions, and holographic characterization of colloidal dynamics. Future studies in these broad research areas undoubtedly will generate further advances in understanding how external forces influence the collective behavior of many-body systems.
