The formation of continuous patterns of nanostructured materials using directed self-assembly under external fields has generated considerable current research interest. We demonstrate for the first time such continuous patterning by inducing irreversible self-assembly leading to nucleation in mesocopic materials (inorganic, organic, and nanoparticles) using a tightly focused laser beam in an optical tweezers apparatus. A dense aqueous dispersion or solution of the material which has high absorption at the laser wavelength is taken in a sample holder so that some material is adsorbed on the top surface. A hot spot is created on the top surface when the adsorbed material absorbs the high intensity at the focus of the laser beam (a submicrometer sized spot), due to which a water vapor bubble is formed. This causes self-assembly of material around the bubble due to Gibbs-Marangoni convection and capillary flow after which the material eventually nucleates into a crystalline state. The bubble is "trapped" at the hot spot due to the temperature gradient around it and can be manipulated by thermal forces generated optically, so that the system may be described as a "thermo-optical" tweezers. We translate the trapped bubble using the microscope sample holder stage of the apparatus so that the nucleation site of the material is simultaneously translated generating continuous patterns. We have demonstrated the technique using exotic inorganic materials such as soft oxometalates, an organic material such as glycine, and a fluorescent dye such as perylene as well as with carbon nanotubes. We have written patterns over lengths of nearly 1 mm at the rate of 1 Hz, with best resolution of about 4 μm. The technique has potential for a wide range of applications ranging from solution processed printable electronics to controlled catalysis.

Cytoskeletal motors drive many essential cellular processes. For example, kinesin-1 transports cargo in a step-wise manner along microtubules. To resolve rotations during stepping, we used optical tweezers combined with an optical microprotractor and torsion balance using highly birefringent microspheres to directly and simultaneously measure the translocation, rotation, force, and torque generated by individual kinesin-1 motors. While, at low adenosine 5'-triphosphate (ATP) concentrations, motors did not generate torque, we found that motors translocating along microtubules at saturating ATP concentrations rotated unidirectionally, producing significant torque on the probes. Accounting for the rotational work makes kinesin a highly efficient machine. These results imply that the motor's gait follows a rotary hand-over-hand mechanism. Our method is generally applicable to study rotational and linear motion of molecular machines, and our findings have implications for kinesin-driven cellular processes.

We use micro-bubble based thermo-optical tweezers to simultaneously synthesize, dope, and pattern conducting polymers to obtain unprecedented conductivity values.

We have designed a supramolecularly bound multi-component catalytic material based on a soft oxometalate (SOM) and a porous organic framework (POF) material, which shows high catalytic conversion efficiency.

The fluctuation-dissipation relation, a central result in non-equilibrium statistical physics, relates equilibrium fluctuations in a system to its linear response to external forces. Here we provide a direct experimental verification of this relation for viscously coupled oscillators, as realized by a pair of optically trapped colloidal particles. A theoretical analysis, in which interactions mediated by slow viscous flow are represented by non-local friction tensors, matches experimental results and reveals a frequency maximum in the amplitude of the mutual response which is a sensitive function of the trap stiffnesses and the friction tensors. This allows for its location and width to be tuned and suggests the utility of the trap setup for accurate two-point microrheology.The relation between the generalized susceptibility and equilibrium fluctuations of the generalized forces, first obtained for a linear resistive circuit by Nyquist [1] and then proved for any general linear dissipative system by Callen and Welton [2], is a central result in non-equilibrium statistical physics. The relation can be used to infer the intrinsic fluctuations of a system from measurements of its response to external perturbations or, perhaps more startlingly, to predict its response to external perturbations from the character of its intrinsic fluctuations [3]. The fluctuation-dissipation relation is the point of departure for several areas of current research including fluctuation relations [4], relaxation in glasses [5], and response and correlations in active [6] and driven systems [7,8].The first experimental verification of the relation between fluctuation and dissipation was due to Johnson [9], whose investigation of the "thermal agitation of electricity in conductors" provided the motivation for Nyquist's theoretical work [1]. Though the relation has been verified since in systems with conservative couplings, a direct verification in a system where the coupling is entirely dissipative is, to the best of our knowledge, not available. Colloidal particles in a viscous fluid interact through velocity-dependent many-body hydrodynamic forces whose strength, away from boundaries, is inversely proportional to the distance between the particles. The range of these dissipative forces can be made much greater than that of conservative forces such as the DLVO interaction [10,11]. Therefore, it is possible to engineer a situation where the dominant coupling between colloidal particles is the viscous hydrodynamic force and all other interactions are negligibly small. Such systems, then, are ideal for testing the fluctuation-dissipation relation when couplings are purely dissipative.In this Letter, we present a direct verification of the fluctuation-dissipation relation for a pair of optically trapped colloidal particles in water. We measure the equilibrium fluctuations of the distance between the particles and the response of one particle to the sinusoidal motion of another particle. Transforming both correlations and responses to the fr...