Optical tweezers use the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometres to tens of micrometres. Since their introduction in 1986, the optical tweezer has become an important tool for research in the fields of biology, physical chemistry and soft condensed matter physics. Recent advances promise to take optical tweezers out of the laboratory and into the mainstream of manufacturing and diagnostics; they may even become consumer products. The next generation of single-beam optical traps offers revolutionary new opportunities for fundamental and applied research.

http://physics.nyu.edu/grierlab/nreview2c/nreview2c.pdf

A revolution in optical manipulation

This review summarizes theoretical progress in the field of active matter, placing it in the context of recent experiments. This approach offers a unified framework for the mechanical and statistical properties of living matter: biofilaments and molecular motors in vitro or in vivo, collections of motile microorganisms, animal flocks, and chemical or mechanical imitations. A major goal of this review is to integrate several approaches proposed in the literature, from semimicroscopic to phenomenological. In particular, first considered are ``dry'' systems, defined as those where momentum is not conserved due to friction with a substrate or an embedding porous medium. The differences and similarities between two types of orientationally ordered states, the nematic and the polar, are clarified. Next, the active hydrodynamics of suspensions or ``wet'' systems is discussed and the relation with and difference from the dry case, as well as various large-scale instabilities of these nonequilibrium states of matter, are highlighted. Further highlighted are various large-scale instabilities of these nonequilibrium states of matter. Various semimicroscopic derivations of the continuum theory are discussed and connected, highlighting the unifying and generic nature of the continuum model. Throughout the review, the experimental relevance of these theories for describing bacterial swarms and suspensions, the cytoskeleton of living cells, and vibrated granular material is discussed. Promising extensions toward greater realism in specific contexts from cell biology to animal behavior are suggested, and remarks are given on some exotic active-matter analogs. Last, the outlook for a quantitative understanding of active matter, through the interplay of detailed theory with controlled experiments on simplified systems, with living or artificial constituents, is summarized.

Hydrodynamics of soft active matter

I. the origin of species by means of natural selection

Entangled quantum states are not separable, regardless of the spatial separation of their components This is a manifestation of an aspect of quantum mechanics known as quantum non-locality An important consequence of this is that the measurement of the state of one particle in a two-particle entangled state defines the state of the second particle instantaneously, whereas neither particle possesses its own well-defined state before the measurement Experimental realizations of entanglement have hitherto been restricted to two-state quantum systems, involving, for example, the two orthogonal polarization states of photons Here we demonstrate entanglement involving the spatial modes of the electromagnetic field carrying orbital angular momentum As these modes can be used to define an infinitely dimensional discrete Hilbert space, this approach provides a practical route to entanglement that involves many orthogonal quantum states, rather than just two Multi-dimensional entangled states could be of considerable importance in the field of quantum information, enabling, for example, more efficient use of communication channels in quantum cryptography

Entanglement of the orbital angular momentum states of photons

Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below. At this scale, inertia is unimportant and the Reynolds number is small. Our emphasis is on the simple physical picture and fundamental flow physics phenomena in this regime. We first give a brief overview of the mechanisms for swimming motility, and of the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming such as resistance matrices for solid bodies, flow singularities and kinematic requirements for net translation. Then we review classical theoretical work on cell motility, in particular early calculations of swimming kinematics with prescribed stroke and the application of resistive force theory and slender-body theory to flagellar locomotion. After examining the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers and the optimization of locomotion strategies. (Some figures in this article are in colour only in the electronic version) This article was invited by Christoph Schmidt.

/pdf/the-hydrodynamics-of-swimming-microorganisms-bonwl3c7w2.pdf

The hydrodynamics of swimming microorganisms

Specific inhibitory reactions of herbicides with photosynthetic reaction centers bound to working electrodes were monitored in a conventional electrochemical cell and a newly designed microfluidic electrochemical flow cell. In both cases, the bacterial reaction centers were bound to a transparent conductive metal oxide, indium-tin-oxide, electrode through carbon nanotubes. In the conventional cell, photocurrent densities of up to a few μA/cm2 could be measured routinely. The photocurrent could be blocked by the photosynthetic inhibitor terbutryn (I
 50 = 0.38 ± 0.14 μM) and o-phenanthroline (I
 50 = 63.9 ± 12.2 μM). The microfluidic flow cell device enabled us to reduce the sample volume and to simplify the electrode arrangement. The useful area of the electrodes remained the same (ca. 2 cm2), similar to the classical electrochemical cell; however, the size of the cell was reduced considerably. The microfluidic flow control enabled us monitoring in real time the binding/unbinding of the inhibitor and cofactor molecules at the secondary quinone site.

Sensing photosynthetic herbicides in an electrochemical flow cell.

Summary form only given. We describe a method which we think is most promising for the building of complex micromechanical devices, where mechanical, optical and electrical components can be readily integrated. The method is based on the photopolymerisation of light curing resins. If polymerisation is performed by a focused laser beam and the reaction is initiated by two-photon excitation, the volume in which the polymerisation reaction proceeds can be very small, i.e. high spatial resolution can be achieved. By moving the focus along a predetermined trajectory, objects of arbitrary shape can be built. The force exerted by focused laser light of the order of 10 mW intensity upon micrometer-size objects has also the right magnitude to move them, i.e. the mechanical systems can be activated by light.

Complex micromachines produced and driven by light

Procedures and methodologies used in cell biophysics have been improved tremendously with the revolutionary advances witnessed in the micro- and nanotechnology in the last two decades. With the advent of microfluidics it became possible to reduce laboratory-sized equipment to the scale of a microscope slide allowing massive parallelization of measurements with extremely low sample volume at the cellular level. Optical micromanipulation has been used to measure forces or distances or to alter the behavior of biological systems from the level of DNA to organelles or entire organisms. Among the main advantages is its non-invasiveness, giving researchers an invisible micro-hand to "touch" or "feel" the system under study, its freely and very often quickly adjustable experimental parameters such as wavelength, optical power or intensity distribution. Atomic force microscopy (AFM) opened avenues for in vitro biological applications concerning with single molecule imaging, cellular mechanics or morphology. As it can operate in liquid environment and at human body temperature, it became the most reliable and accurate nanoforce-tool in the research of cell biophysics. In this paper we review how the above three techniques help increase our knowledge in biophysics at the cellular level.

Micro- and nanotechnology for cell biophysics

Optical micromanipulation in its original form involves the trapping of micrometer sized spheres in a high numerical aperture focused laser light. This arrangement permits the control of the location of the trapped sphere – with appropriate modifications the procedure proved to be a very useful micromanipulation tool. The capabilities of optical trapping can be vastly enhanced if instead of a sphere a complex structure, and instead of the simple focused light a shaped light pattern is used: this way most complicated manipulation schemes can be realized. The trapped objects can be rotated, they can be used like optical rotators, microprobes can be precisely directed to points of interest without mechanical contact, light driven micromechanical devices can be constructed to perform different tailored tasks. With appropriate surface modifications the optically manipulated microtools can be applied on microscopic test objects for mechanical actuation, spectroscopic investigations, etc. The possibilities are endless. In this article we will introduce the basic concepts of the technology: microstructure building by laser induced photopolymerization, the selective functionalization of the microtool surfaces, and the controlled optical micromanipulation with shaped optical fields. We will describe typical applications in diverse fields, like light driven micromechanical machines, light actuated microtools for local spectroscopy, technique for the torsional manipulation of biological macromolecules. The examples will illustrate the immense potential of the approach in the investigation and manipulation of microscopic objects.

Microstructures Created and Driven by Light: Extending the Capabilities of Optical Manipulation by Using Structures of Complex Shape and Patterned Optical Fields

Chlamydomonas reinhardtii is a model organism of increasing biotechnological importance, yet, the evaluation of its life cycle processes and photosynthesis on a single-cell level is largely unresolved. To facilitate the study of the relationship between morphology and photochemistry, we established microfluidics in combination with chlorophyll a fluorescence induction measurements. We developed two types of microfluidic platforms for single-cell investigations: (i) The traps of the “Tulip” device are suitable for capturing and immobilizing single cells, enabling the assessment of their photosynthesis for several hours without binding to a solid support surface. Using this “Tulip” platform, we performed high-quality non-photochemical quenching measurements and confirmed our earlier results on bulk cultures that non-photochemical quenching is higher in ascorbate-deficient mutants (Crvtc2-1) than in the wild-type. (ii) The traps of the “Pot” device were designed for capturing single cells and allowing the growth of the daughter cells within the traps. Using our most performant “Pot” device, we could demonstrate that the FV/FM parameter, an indicator of photosynthetic efficiency, varies considerably during the cell cycle. Our microfluidic devices, therefore, represent versatile platforms for the simultaneous morphological and photosynthetic investigations of C. reinhardtii on a single-cell level.

/pdf/microfluidic-platforms-designed-for-morphological-and-1eoai74x.pdf

Peter Galajda

Papers

Sensing photosynthetic herbicides in an electrochemical flow cell.

Complex micromachines produced and driven by light

Micro- and nanotechnology for cell biophysics

Microstructures Created and Driven by Light: Extending the Capabilities of Optical Manipulation by Using Structures of Complex Shape and Patterned Optical Fields

Microfluidic Platforms Designed for Morphological and Photosynthetic Investigations of Chlamydomonas reinhardtii on a Single-Cell Level