Rotational control within optical tweezers by use of a rotating aperture

Gradient torque and its effect on rotational dynamics of optically trapped non-spherical particles in the elliptic Gaussian beam

Rotational motion of the optically trapped particle is a topic of enduring interest, while the changes of angular velocity in one rotation period remain largely unexplored. Here, we proposed the optical gradient torque in the elliptic Gaussian beam, and the instantaneous angular velocities of alignment and fluctuant rotation of the trapped non-spherical particles are investigated for the first time. The fluctuant rotations of optically trapped particles are observed, and the angular velocity fluctuated twice per rotation period, which can be used to determine the shape of trapped particles. Meanwhile, a compact optical wrench is invented based on the alignment, and its torque is adjustable and is larger than the torque of a linearly polarized wrench with the same power. These results provide a foundation for precisely modelling the rotational dynamics of optically trapped particles, and the presented wrench is expected to be a simple and practical micro-manipulating tool.

Rotational manipulation of a microscopic object inside a microfluidic channel

Observations and analyses of a microscopic object are essential processes in various fields such as chemical engineering and life science. Microfluidic techniques with various functions and extensions have often been used for such purposes to investigate the mechanical properties of microscopic objects such as biological cells. One of such extensions proposed in this context is a real-time visual feedback manipulation system, which is composed of a high-speed camera and a piezoelectric actuator with a single-line microfluidic channel. Although the on-chip manipulation system enables us to control the 1 degree-of-freedom position of a target object by the real-time pressure control, it has suffered from unintended changes in the object orientation, which is out of control in the previous system. In this study, we propose and demonstrate a novel shear-flow-based mechanism for the control of the orientation of a target object in addition to the position control in a microchannel to overcome the problem of the unintended rotation. We designed a tributary channel using a three-dimensional hydrodynamic simulation with boundary conditions appropriate for the particle manipulation to apply shear stress to the target particle placed at the junction and succeeded in rotating the particle at an angular velocity of 0.2 rad/s even under the position control in the experiment. The proposed mechanism would be applied to feedback controls of a target object in a microchannel to be in a desired orientation and at a desired position, which could be a universally useful function for various microfluidic platforms.

Demonstration of a simple technique for controllable revolution of light-absorbing particles in air

The rotation of optically trapped particles is used in many applications for the realization of different micromechanical devices, such as micropumps, microrotors, and microgyroscopes, as well as for the investigation of particle interactions. Although for transparent micro-objects in both liquid media and vacuum, the rotation can easily be realized by transfer of the spin angular or orbital angular momentum from the light to the object. In the case of light-absorbing micro-objects in gaseous media, such transfers are insignificant in comparison with the thermal effects arising from the photo- and thermo-phoresis phenomena initiating the movement of trapped particles in a laser beam. Currently, proposed methods using a single focused laser beam, tapered-ring optical traps, or single and multiple bottle beams (BBs) have various limitations-for example, the inability to control the direction of the revolution of trapped particles or the low revolution frequency and small revolution angles. Here we propose a simple method for the realization of the revolution of airborne light-absorbing particles. The method is based on a combination of a circular diaphragm and a rotating cylindrical lens, enabling the generation of linear optical BBs. Our results show the flexibility and reliability of the proposed technique, allowing such laser traps to be used in various optical systems for the manipulation of micro-objects with different dimensions and shapes.

Man-made rotary nanomotors: a review of recent developments

The development of rotary nanomotors is an essential step towards intelligent nanomachines and nanorobots. In this article, we review the concept, design, working mechanisms, and applications of state-of-the-art rotary nanomotors made from synthetic nanoentities. The rotary nanomotors are categorized according to the energy sources employed to drive the rotary motion, including biochemical, optical, magnetic, and electric fields. The unique advantages and limitations for each type of rotary nanomachines are discussed. The advances of rotary nanomotors is pivotal for realizing dream nanomachines for myriad applications including microfluidics, biodiagnosis, nano-surgery, and biosubstance delivery.

Rotation dynamics of particles trapped in a rotating beam

The rotation dynamics of particles trapped in a rotating beam is theoretically investigated. We find that there is a critical angular speed for the rotating beam. If the angular speed of the rotating beam is smaller than the critical value, the angular velocity of the trapped particle is nearly the same as that of the rotating beam, which is in accord with existing experimental observation. Otherwise, the angular velocity of the trapped particles will become periodic or quasi-periodic with time, depending on the beam polarization, which, to the best of our knowledge, has not been previously reported. Moreover, we also propose some methods to determine the ratio between the beam power and the maximal angular speed of the trapped particle, which can be used to estimate the minimum power required to rotate the particle at a given angular speed.

Rotation, oscillation and hydrodynamic synchronization of optically trapped oblate spheroidal microparticles

While the behavior of optically trapped dielectric spherical particles has been extensively studied, the behavior of non-spherical particles remains mainly unexplored. In this work we focus on the dynamics of oblate spheroidal particles trapped in a tightly focused elliptically-polarized vortex beam. In our experiments we used polystyrene spheroids of aspect ratio of major to minor axes equal to 2.55 and of a volume equal to a sphere of diameter 1.7μm. We demonstrate that such particles can be trapped in three dimensions, with the minor axis oriented perpendicular to both the beam polarization (linear) and the beam propagation, can spin in a circularly polarized beam and an optical vortex beam around the axis parallel with the beam propagation. We also observed that these particles can exhibit a periodic motion in the plane transversal to the beam propagation. We measured that the transfer of the orbital angular momentum from the vortex beam to the spheroid gives rise to torques one order of magnitude stronger comparing to the circularly polarized Gaussian beam. We employed a phase-only spatial light modulator to generate several vortex beam traps with one spheroid in each of them. Due to independent setting of beams parameters we controlled spheroids frequency and sense of rotation and observed hydrodynamic phase and frequency locking of rotating spheroids. These optically driven spheroids offer a simple alternative approach to the former techniques based on birefringent, absorbing or chiral microrotors.

Torque measurement at the single-molecule level

Methods for exerting and measuring forces on single molecules have revolutionized the study of the physics of biology. However, it is often the case that biological processes involve rotation or torque generation, and these parameters have been more difficult to access experimentally. Recent advances in the single-molecule field have led to the development of techniques that add the capability of torque measurement. By combining force, displacement, torque, and rotational data, a more comprehensive description of the mechanics of a biomolecule can be achieved. In this review, we highlight a number of biological processes for which torque plays a key mechanical role. We describe the various techniques that have been developed to directly probe the torque experienced by a single molecule, and detail a variety of measurements made to date using these new technologies. We conclude by discussing a number of open questions and propose systems of study that would be well suited for analysis with torsional measurement techniques.

Optically-actuated translational and rotational motion at the microscale for microfluidic manipulation and characterization

The single beam optical trap (optical tweezers), a highly focused beam, is on its way to revolutionizing not only the fields of colloidal physics and biology, but also materials science and engineering. Recently, spatially-extended three-dimensional light patterns have gained considerable usage for exerting force to alter, manipulate, organize and characterize materials. To advance the degree of manipulation, such as rotation of materials in microfluidic environments along with spatial structuring, other beam parameters such as phase and polarization have to be configured. These advances in optical tweezers' technology have enabled complex microfluidic actuation and sorting. In addition to remotely (in a non-contact way) applying force and torques in three-dimensions, which can be continuously varied unlike mechanical manipulators, optical tweezers-based methods can be used for sensing the force of interaction between microscopic objects in a microfluidic environment and for the characterization of micro-rheological properties. In this review, we place emphasis on applications of optical actuation based on novel beams in performing special functions such as rotation, transportation, sorting and characterization of the microscopic objects. Further, we have an extended discussion on optical actuation (transport and rotation) with fiber optic microbeams and spectroscopic characterization in the microfluidic environment. All these advancements in optical manipulation would further facilitate the growing use of optical tools for complex microfluidic manipulations.

Measurement of the trapping efficiency of an elliptical optical trap with rigid and elastic objects

Optical tweezers and their various modifications offer a sophisticated way to perform noncontact cell manipulation. In this paper, we quantify forces existing in an elliptical trap formed by two cylindrical lenses and compare the results with a point optical trap case. The trapping efficiency of point and elliptical traps was analyzed by measuring the Q values of both traps. Polystyrene microspheres and red blood cells (RBCs) were used as samples. Stretching of the RBC was taken into account in the Q value measurements. Although the Q value of a point optical trap is larger than that of an elliptical trap when measured for a single RBC, we can manipulate the orientation of an RBC in a point trap with the elliptical trap and can also trap several RBCs simultaneously in the elliptical trap far from the cuvette surfaces by using a long-working-distance water immersion objective. This opens new possibilities for studying light-matter interactions at the cellular level.

Trapping and rotating microparticles and bacteria with moiré-based optical propelling beams

We propose and demonstrate trapping and rotation of microparticles and biological samples with a moiré-based rotating optical tweezers. We show that polystyrene beads, as well as Escherichia coli cells, can be rotated with ease, while the speed and direction of rotation are fully controllable by a computer, obviating mechanical movement or phase-sensitive interference. Furthermore, we demonstrate experimentally the generation of white-light propelling beams and arrays, and discuss the possibility of optical tweezing and particle micro-manipulation based on incoherent white-light rotating patterns.

Optical orientation and rotation of trapped red blood cells with Laguerre-Gaussian mode

We report the use of Laguerre-Gaussian (LG) modes for controlled orientation and rotation of optically trapped red blood cells (RBCs). For LG modes with increasing topological charge the resulting increase in size of the intensity annulas led to trapping of the cells at larger tilt angle with respect to the beam axis and thus provided additional control on the stable orientation of the cells under trap. Further, the RBCs could also be driven as micro-rotors by a transfer of orbital angular momentum from the LG trapping beam having large topological charge or by rotating the profile of LG mode having fractional topological charge.

Rotation of single bacterial cells relative to the optical axis using optical tweezers

Using a single-beam, oscillating optical tweezers, we demonstrate trapping and rotation of rod-shaped bacterial cells with respect to the optical axis. The angle of rotation, θ, is determined by the amplitude of the oscillation. It is shown that θ can be measured from the longitudinal cell intensity profiles in the corresponding phase-contrast images. The technique allows viewing the cell from different perspectives and can provide a useful tool in fluorescence microscopy for the analysis of three-dimensional subcellular structures.

Stable trapping and manually controlled rotation of an asymmetric or birefringent microparticle using dual-mode split-beam optical tweezers

Inserting a coverslip into half of a Gaussian laser beam at a suitable tilting angle can make the single-mode laser beam become closely spaced dual light spots at the laser focus. In this way, we can reform the conventional single-beam optical tweezers easily and construct a set of dual-mode split-beam optical tweezers, which can be used to manually rotate a trapped and twisted red blood cell around the optical axis. Furthermore, we demonstrate that the split-beam optical tweezers can also stably trap and orient a birefringent polystyrene micro strip particle, which otherwise will self rotate at a varying speed along the structural principal axes, fast spin about the optical axis in a tilting pose, or precess like a gyroscope, in the original linearly polarized single-beam optical tweezers.

Full 3D translational and rotational optical control of multiple rod-shaped bacteria

The class of rod-shaped bacteria is an important example of non-spherical objects where defined alignment is desired for the observation of intracellular processes or studies of the flagella. However, all available methods for orientational control of rod-shaped bacteria are either limited with respect to the accessible rotational axes or feasible angles or restricted to one single bacterium. In this paper we demonstrate a scheme to orientate rod-shaped bacteria with holographic optical tweezers (HOT) in any direction. While these bacteria have a strong preference to align along the direction of the incident laser beam, our scheme provides for the first time full rotational control of multiple bacteria with respect to any arbitrary axis. In combination with the translational control HOT inherently provide, this enables full control of all three translational and the two important rotational degrees of freedom of multiple rod-shaped bacteria and allows one to arrange them in any desired configuration.

Controllable rotation of optical beams with bored helical phases

We achieve controllable noninterferometric rotation of a bored helical beam by introducing a phase shift exclusively to the annular helical region of the phase. We present a derivation based on the decomposition of the beams, which shows that a constant phase shift of DeltaPhi between the bore and the surrounding helical phase with topological charge l will rotate the intensity profile by -DeltaPhi/l about its center. The effect of the phase shifting is verified with experiments. This technique is simple, while it preserves the transverse intensity profiles of the beams. Our report may find applications in optical manipulation and trapping.

Rotational dynamics of optically trapped nanofibers

We report on the experimental evidence of tilted polymer nanofiber rotation, using a highly focused linear polarized Gaussian beam. Torque is controlled by varying trapping power or fiber tilt angle. This suggests an alternative strategy to previously reported approaches for the rotation of nano-objects, to test fundamental theoretical aspects. We compare experimental rotation frequencies to calculations based on T-Matrix formalism, which accurately reproduces measured data, thus providing a comprehensive description of trapping and rotation dynamics of the linear nanostructures.

Rotation of birefringent particles in optical tweezers with spherical aberration

Birefringent particles rotate when trapped in elliptically polarized light. When an infinity corrected oil-immersion objective is used for trapping, rotation of birefringent particles in optical tweezers based on an infinity optical microscope is affected by the spherical aberration at the glass-water interface. The maximum rotation rate of birefringent particles occurs close to the coverslip, and the rotation rate decreases dramatically as the trapped depth increases. We experimentally demonstrate that spherical aberration can be compensated by using a finite-distance-corrected objective to trap and rotate the birefringent particles. It is found that the trapped depth corresponding to the maximum rotation rate is 50 microm, and the rotation rates at deep trapped depths are improved.

High rotation speed of single molecular microcrystals in an optical trap with elliptically polarized light

We build an experiment of optical tweezers based on the use of an inverted optical microscope for manipulating microsized single crystals, which are made of an organic dye and parallelepiped in shape. The microcrystals are directed so that their long axis is in the axial direction of the trapping beam. Their short axis follows the direction of the linear polarization of the beam. In circular or elliptic polarization, the crystals are spontaneously put in rotation with a high speed of up to 500 turns per second. It is the first time, to the best of our knowledge, that such a result is reported for particles of the size of our crystals. Another surprising result is that the rotation speed was first increased as expected by increasing the incident power, but after passing by a maximum it decreased until the complete stop of rotation, whereas the power continued growing. This evolution was not reversible. Several hypotheses are discussed to explain such behavior.

Light at work: the use of optical forces for particle manipulation, sorting, and analysis

We review the combinations of optical micro-manipulation with other techniques and their classical and emerging applications to non-contact optical separation and sorting of micro- and nanoparticle suspensions, compositional and structural analysis of specimens, and quantification of force interactions at the microscopic scale. The review aims at inspiring researchers, especially those working outside the optical micro-manipulation field, to find new and interesting applications of these methods.

The optical cell rotator

The optical cell rotator (OCR) is a modified dual-beam laser trap for the holding and controlled rotation of suspended dielectric microparticles, such as cells. In contrast to optical tweezers, OCR uses two counter-propagating divergent laser beams, which are shaped and delivered by optical fibers. The rotation of a trapped specimen is carried out by the rotation of a dual-mode fiber, emitting an asymmetric laser beam. Experiments were performed on human erythrocytes, promyelocytic leukemia cells (HL60), and cell clusters (MCF-7). Since OCR permits the rotation of cells around an axis perpendicular to the optical axis of any microscope and is fully decoupled from imaging optics, it could be a suitable and expedient tool for tomographic microscopy.

Independent polarisation control of multiple optical traps

We present a system which uses a single spatial light modulator to control the spin angular momentum of multiple optical traps. These traps may be independently controlled both in terms of spatial location and in terms of their spin angular momentum content. The system relies on a spatial light modulator used in a "split-screen" configuration to generate beams of orthogonal polarisation states which are subsequently combined at a polarising beam splitter. Defining the phase difference between the beams with the spatial light modulator enables control of the polarisation state of the light. We demonstrate the functionality of the system by controlling the rotation and orientation of birefringent vaterite crystals within holographic optical tweezers.

A mechanism of polarized light sensitivity in cone photoreceptors of the goldfish Carassius auratus

An integrated laser tweezer and microphotometry device has been used to characterize in detail how individual, axially orientated goldfish photoreceptors absorb linearly polarized light. This work demonstrates that the mid-wavelength sensitive members of double cone photoreceptors display axial differential polarization sensitivity. The polarization contrast was measured to be 9.2 +/- 0.4%. By comparison, rod photoreceptors only exhibit isotropic absorbance. These data, combined with the square cone mosaic of double cones in the retina, suggest that intrinsic axial dichroism forms part of the underlying biophysical detection mechanism for polarization vision in this species.

2D optical manipulation and assembly of shape-complementary planar microstructures

Optical trapping and manipulation offer great flexibility as a non-contact microassembly tool. Its application to the assembly of microscale building blocks may open new doors for micromachine technology. In this work, we demonstrate all-optical assembly of microscopic puzzle pieces in a fluidic environment using programmable arrays of trapping beams. Identical shape-complimentary pieces are optically fabricated with submicron resolution using two-photon polymerization (2PP) technique. These are efficiently assembled into space-filling tessellations by a multiple-beam optical micromanipulation system. The flexibility of the system allows us to demonstrate both user-interactive and computer-automated modes of serial and parallel assembly of microscale objects with high spatial and angular positioning precision.

A programmable optical angle clamp for rotary molecular motors

Optical tweezers are widely used for experimental investigation of linear molecular motors. The rates and force dependence of steps in the mechanochemical cycle of linear motors have been probed giving detailed insight into motor mechanisms. With similar goals in mind for rotary molecular motors we present here an optical trapping system designed as an angle clamp to study the bacterial flagellar motor and F(1)-ATPase. The trap position was controlled by a digital signal processing board and a host computer via acousto-optic deflectors, the motor position via a three-dimensional piezoelectric stage and the motor angle using a pair of polystyrene beads as a handle for the optical trap. Bead-pair angles were detected using back focal plane interferometry with a resolution of up to 1 degrees , and controlled using a feedback algorithm with a precision of up to 2 degrees and a bandwidth of up to 1.6 kHz. Details of the optical trap, algorithm, and alignment procedures are given. Preliminary data showing angular control of F(1)-ATPase and angular and speed control of the bacterial flagellar motor are presented.

Optical Torque on Microscopic Objects

We outline in general the role and potential areas of application for the use of optical torque in optical tweezers. Optically induced torque is always a result of transfer of angular momentum from light to a particle with conservation of momentum as an underlying principle. Consequently, rotation can be induced by a beam of light that carries angular momentum (AM) or by a beam that carries no AM but where AM is induced in the beam by the particle. First, we analyze some techniques to exert torque with optical tweezers such as dual beam traps. We also discuss the alignment and rotation which is achieved using laser beams carrying intrinsic AM-either spin or orbital AM, or both. We then discuss the types of particles that can be trapped and rotated in such beams such as absorbing or birefringent particles. We present a systematic study of the alignment of particles with respect to the beam axis and the beam's polarization as a way of inducing optical torque by studying crystals of the protein lysozyme. We present the theory behind quantitative measurements of both spin and orbital momentum transfer. Finally, we discuss the applications of rotation in optically driven micromachines, microrheology, flow field measurements, and microfluidics.

Optical Micromanipulation Methods for Controlled Rotation, Transportation, and Microinjection of Biological Objects

The use of laser microtools for rotation and controlled transport of microscopic biological objects and for microinjection of exogenous material in cells is discussed. We first provide a brief overview of the laser tweezers-based methods for rotation or orientation of microscopic objects. Particular emphasis is placed on the methods that are more suitable for the manipulation of biological objects, and the use of these for two-dimensional (2D) and 3D rotations/orientations of intracellular objects is discussed. We also discuss how a change in the shape of a red blood cell (RBC) suspended in hypertonic buffer leads to its rotation when it is optically tweezed. The potential use of this approach for the diagnosis of malaria is also illustrated. The use of a line tweezers having an asymmetric intensity distribution about the center of its major axis for simultaneous transport of microscopic objects, and the successful use of this approach for induction, enhancement, and guidance of neuronal growth cones is presented next. Finally, we describe laser microbeam-assisted microinjection of impermeable drugs into cells and also briefly discuss possible adverse effects of the laser trap or microbeams on cells.

Direct measurement of torque in an optical trap and its application to double-strand DNA

We present a method that offers the possibility to directly apply and measure torque on particles in an optical trap. It can be used to rotationally manipulate biopolymers attached to appropriate particles. A flat object is trapped and oriented in the focus of a linearly polarized laser light. The direction and power of the orientational trap are controlled by the polarization state of the light. As a demonstration of the capabilities of the method, we examined the torsional stiffness of dsDNA (lambda-DNA) in its linear torsional regime by directly measuring the torque generated by the molecule.

Integrated optical motor

A light-driven micrometer-sized mechanical motor is created by laser-light-induced two-photon photopolymerization. All necessary components of the engine are built upon a glass surface by an identical procedure and include the following: a rigid mechanical framework, a rotor freely rotating on an axis, and an integrated optical waveguide carrying the actuating light to the rotor. The resulting product is a most practical stand-alone system. The light introduced into the integrated optical waveguide input of the motor provides the driving force: neither optical tweezers or even a microscope are needed for the function. The power and efficiency of the motor are evaluated. The independent unit is expected to become an important component of more complex integrated lab-on-a-chip devices.

Manipulation and growth of birefringent protein crystals in optical tweezers

We report on the trapping, rotation, and in-situ growth of birefringent tetragonal lysozyme crystals in optical tweezers operating at a wavelength of 1070 nm. Variation of the pH and lysozyme concentration of the solution during growth was used to alter the length to width ratio of the crystals, and hence their orientation in the tweezers. Crystals with the optical axis skewed or perpendicular to the trapping-beam axis could be rotated by changing the orientation of linearly polarized light. We observed spontaneous spinning of some asymmetric crystals in the presence of linearly polarized light, due to radiation pressure effects. Addition of protein to the solution in the tweezers permitted real-time observation of crystal growth.

Optical torque wrench: angular trapping, rotation, and torque detection of quartz microparticles

We describe an apparatus that can measure the instantaneous angular displacement and torque applied to a quartz particle which is angularly trapped. Torque is measured by detecting the change in angular momentum of the transmitted trap beam. The rotational Brownian motion of the trapped particle and its power spectral density are used to determine the angular trap stiffness. The apparatus features a feedback control that clamps torque or other rotational quantities. The torque sensitivity demonstrated is ideal for the study of known biological molecular motors.

Optical microrheology using rotating laser-trapped particles

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.

Spatial light modulator-controlled alignment and spinning of birefringent particles optically trapped in an array

We demonstrate the use of a phase-only liquid-crystal spatial light modulator (SLM) for polarization-controlled rotation and alignment of an array of optically trapped birefringent particles. A collimated beam incident upon a two-dimensional lenslet array yields multiple foci, scaled to produce optical gradient traps with efficient three-dimensional trapping potentials. The state of polarization of each trapping beam is encoded by the SLM, which acts as a matrix of wave plates with computer-controlled phase retardations. Control of the rotation frequency and alignment direction of the particles is achieved by the transfer of tunable photon spin angular momentum.

Optical doughnut for optical tweezers

We describe novel optical doughnuts for optical tweezers. With new phase functions, the proposed doughnut beams have dark cores in specified shapes. The technique can offer a simple method for creating a variety of beam shapes to match the trapped objects. One can rotate the beams directly by revolving their phase structures about their axes on the initial plane. The technique for generating the traditional Laguerre-Gaussian beam can be used to create these novel beams.

Interactive light-driven and parallel manipulation of inhomogeneous particles

A light-driven micromanipulation system with real-time userfeedback control is used to simultaneously trap colloidal suspensions enabling a unique interactive sorting capability and arbitrary patterning of microscopic particles. The technique is based on a straightforward phase-tointensity conversion generating multiple beam patterns for manipulation of particles in the observation plane of a microscope. Encoding of phase patterns in a spatial light modulator, which is directly controlled by a computer, allows for dynamic reconfiguration of the trapping patterns, where independent control of the position, size, shape and intensity of each beam is possible. Efficient sorting of microsphere mixtures of distinct sizes and colors using multiple optical traps is demonstrated.

Light torque nanocontrol, nanomotors and nanorockers

In a novel application of light torques, we manipulate and control the rotation of nanorods. We apply light torques to 250 nm diameter glass nanorods in a single-beam optical trap. Light-torque operated nanomotors whir at moderate speeds that depend on several factors, including the magnitude of the light torque, the viscosity of the surrounding medium, and the rotation rate of the electric field vector of the linearly polarized trapping light. Two new modes of behavior - rocking motion and saltatory motion - are also described and explained.

Optical angular momentum transfer to transparent isotropic particles using laser beam carrying zero average angular momentum

The torque exerted by an astigmatic optical beam on small transparent isotropic particles was dynamically measured observing the angular motion of the particles under a microscope. The data confirmed that torque was originated by the transfer of orbital angular momentum associated with the spatial changes in the phase of the optical field induced by the moving particle. This mechanism for angular momentum transfer works also with incident light beams with no net angular momentum.