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Manas Khan

Indian Institute of Technology, Kanpur, Department of Physics, Faculty Member

Indian Institute of Science, Physics Department, Alumnus

University of California, Los Angeles, Chemistry and Biochemistry, Alumnus

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I use Optical Tweezers as a tool to manipulate micron and sub-micron particles. The goal is to investigate various aspects of soft matters (colloidal suspensions, surfactant gels etc) while the interest is in understanding statistical physics.

Education :

M.S. (Physics) :
Indian Institute of Science, Bangalore, 2003
B.Sc. (Physics) :
Presidency College, Kolkata, 2000

Non-Academic Interest :

Wildlife and Nature Photography
Wildlife and Nature Conservation
Phone: 7677476776
Address: Department of Physics, IIT Kanpur
Indian Institute of Technology Kanpur

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Books by Manas Khan

Optical Micromanipulation of Soft Materials: Applications in Devices and Technologies

Soft Materials for Functional Applications, 2025

Since its invention by Arthur Ashkin and colleagues at Bell Labs in the 1970s, optical micromanip... more Since its invention by Arthur Ashkin and colleagues at Bell Labs in the 1970s, optical micromanipulation, also known as optical tweezers or laser tweezers, has evolved remarkably to become one of the most convenient and versatile tools for studying soft materials, including biological systems. Arthur Ashkin received the Nobel Prize in Physics in 2018 for enabling these extraordinary scientific advancements. Essentially, a focused laser beam is used to apply and measure minuscule forces from a few piconewtons to femtonewtons by utilizing light-matter interaction at mesoscopic length scales. Combined with advanced microscopy and position-sensing techniques, optical micromanipulations enable us to investigate diverse aspects of functional soft materials. These include studying mechanical responses through force-elongation measurements, examining the structural properties of complex fluids employing microrheology, analyzing chemical compositions using spectroscopy, and sorting cells through single-cell analysis. Furthermore, it is utilized in various soft-matterbased devices, such as laser scissors and optical motors in microfluidic channels. This chapter presents an overview of optical micromanipulation techniques by describing fundamental theories and explaining the design considerations of conventional single-trap and dual-trap setups as well as recent improvisations. We further discuss their capabilities and applications in probing exotic soft-matter systems and in developing widely utilized devices and technologies based on functional soft materials.

Optical Tweezers: Methods and Applications in Contemporary Research

Optical Tweezers to Probe and Manipulate Soft, Nano and Bio Systems

$FIGURE 1.2: The origin of the gradient and scattering force for a particle in Mie regime. (a) As an incident ray gets refracted through the particle, the change in momentum is transferred to the particle in the form of a reaction force. (b) Depending on the position of the particle in the intensity gradient of the light beam the net force changes, nevertheless it always pulls the particle to the point of highest intensity and zero intensity gradient. (c) The scattering force always pushes the particle downstream, independent of its position.$

$FIGURE 1.4: The force exerted by an incident ray of power P at an angle of incidence 6 on a dielectric particle. Its components that get reflected and refracted at the surface of the particle are shown with relative power through color codes. The light beam is propagating along Z.$

$FIGURE 6.5: Time sequences of different sized and shaped Al2O3 rotors are shown here. In each time frame the orientation of the rotor is indicated by an arrow. The rotor in panel A is a typical nanorod whereas the rotor in panel B is a bigger asymmetric nanorod and in panel C, the rotor is an odd nanorod bundle. The structures of the rotors in panel B and C have been depicted as schematics in the rightmost column. Magnification factors of all the images are same. The images shown in panel A are diffraction limited images and hence they do not convey the real size of the rotor. in the infrared wavelengths.$

$FIGURE 6.6: Rotational motion of a small ZnO rotor is displayed here. The arrow indicates the orientation of the rotor. The images are diffraction limited images and therefore they do not convey the real size of the rotor. of a small ZnO rotor rotating along the counter clock-wise direction. Movie files showing$

$FIGURE 6.8: Panel A shows the rotation sequences of a MgO nanorod trapped vary close to the bottom plate. The rotor rotated horizontally at 2Hz. Panel B and C display rotations of two Si nanorotors. The images in these cases are diffraction limited and hence they do not represent the real sizes of the rotors.$

$Optical Trapping and Transportation of Pd-decorated SWNT FIGURE 7.4: Optical microscope images of trapped Pd-SWNT (a-c) and pure SWNT (d). In frame (a) the laser is off. Frame (b) shows the trapping of Pd-SWNT at 118 mW laser power. The image is a diffraction limited image and hence it does not represent the real size of the trapped nanotube. In (c), the trapping of Pd-SWNT has been shown at laser power of 170 mW when more than one nanotube get trapped and a more prominent dark spot is visible at the trap center. Frame (d) shows the trapping of pure SWNT at 214 mW.$

Papers by Manas Khan

Dynamically stable optical trapping of thermophoretically active Janus colloids

ACS Photonics, 2026

The ability to optically trap and manipulate arti cial microswimmers such as active Janus particl... more The ability to optically trap and manipulate arti cial microswimmers such as active Janus particles (JPs) provides a breakthrough in active matter research and applications. However, it presents signi cant challenges because of the asymmetry in the optical properties of JPs and remains incomprehensible. Illustrating the interplay between optical and thermophoretic forces, we demonstrate dynamically stable optical trapping of Pt-silica JPs, where the forcebalanced position evolves spontaneously within a localized volume around the focal point and in a vertically shifted annular con nement at low and high laser powers, respectively. Intriguingly, the orientational and orbital dynamics of JP remain strongly coupled in the delocalized con nement. Furthermore, we demonstrate simultaneous optical trapping of multiple JPs. This rst report on thermophoresis of Pt-silica JPs and localized-to-delocalized crossover in the position distributions of an optically trapped active JP, verifying theoretical predictions, advances our understanding on con ned active matter and their experimental realizations.

Local collective motion analysis for multi-probe dynamic imaging and microrheology

Journal of physics. Condensed matter : an Institute of Physics journal, Aug 7, 2016

Dynamical artifacts, such as mechanical drift, advection, and hydrodynamic flow, can adversely af... more Dynamical artifacts, such as mechanical drift, advection, and hydrodynamic flow, can adversely affect multi-probe dynamic imaging and passive particle-tracking microrheology experiments. Alternatively, active driving by molecular motors can cause interesting non-Brownian motion of probes in local regions. Existing drift-correction techniques, which require large ensembles of probes or fast temporal sampling, are inadequate for handling complex spatio-temporal drifts and non-Brownian motion of localized domains containing relatively few probes. Here, we report an analytical method based on local collective motion (LCM) analysis of as few as two probes for detecting the presence of non-Brownian motion and for accurately eliminating it to reveal the underlying Brownian motion. By calculating an ensemble-average, time-dependent, LCM mean square displacement (MSD) of two or more localized probes and comparing this MSD to constituent single-probe MSDs, we can identify temporal regimes dur...

Optical Tweezers: Methods and Applications in Contemporary Research

Trajectories of probe spheres in generalized linear viscoelastic complex fluids

Soft matter, Jan 7, 2014

We have developed a fast simulation that generates a random walk of an isolated probe sphere in a... more We have developed a fast simulation that generates a random walk of an isolated probe sphere in a generalized linear viscoelastic complex fluid over a highly extended dynamic range. We introduce a coupled harmonically bound Brownian particle (c-HBBP) model, in which the relaxation modes of the viscoelastic medium are treated as harmonic wells. These wells are coupled to the probe sphere and perform Brownian motion in bound harmonic potentials corresponding to the next-longer relaxation mode, according to the relaxation spectrum of the viscoelastic material. We implement this c-HBBP model by generating variable temporal step sizes that have a uniform distribution in logarithmic time. We create and analyze trajectories for several different viscoelastic complex fluids: a polymer system at its gel point, a dense emulsion system, a blend of two monodisperse polystyrene polymers for which the relaxation spectrum has been measured, and a model anisotropic soft system that shows dense emul...

Tunable Brownian vortex at the interface

Physical Review E, 2011

A general kind of Brownian vortices is demonstrated by applying an external nonconservative force... more A general kind of Brownian vortices is demonstrated by applying an external nonconservative force field to a colloidal particle bound by a conservative optical trapping force at a liquid-air interface. As the liquid medium is translated at a constant velocity with the bead trapped at the interface, the drag force near the surface provides enough rotational component to bias the particle's thermal fluctuations in a circulatory motion. The interplay between the thermal fluctuations and the advection of the bead in constituting the vortex motions is studied, and we infer that the angular velocity of the circulatory motion offers a comparative measure of the interface fluctuations.

Optical trapping and transportation of carbon nanotubes made easy by decorating with palladium

Optics Express, 2006

Individual carbon nanotubes being substantially smaller than the wavelength of light, are not muc... more Individual carbon nanotubes being substantially smaller than the wavelength of light, are not much responsive to optical manipulation. Here we demonstrate how decorating single-walled carbon nanotubes with palladium particles makes optical trapping and manipulation easier. Palladium decorated nanotubes (Pd/SWNTs) have higher effective dielectric constant and are trapped at much lower laser power level with greater ease. In addition, we report the transportation of Pd/SWNTs using an asymmetric line trap. Using this method carbon nanotubes can be transported in any desired direction with high transportation speed.

Nanorotors using asymmetric inorganic nanorods in an optical trap

Nanotechnology, 2006

We demonstrate how light force, irrespective of the polarization of the light, can be used to run... more We demonstrate how light force, irrespective of the polarization of the light, can be used to run a simple nanorotor. While the gradient force of a single beam optical trap is used to hold an asymmetric nanorod, we utilize the scattering force to generate a torque on the nanorod, making it rotate about the optic axis. The inherent textural irregularities or morphological asymmetries of the nanorods give rise to the torque under the radiation pressure. Even a small surface irregularity with non-zero chirality is sufficient to produce enough torque for moderate rotational speed. Different sized rotors can be used to set the speed of rotation over a wide range with fine tuning possible through the variation of the laser power. We present a simple dimensional analysis to qualitatively explain the observed trend of the rotational motion of the nanorods.

Optically Driven Nanorotors: Experiments and Model Calculations

Journal of Nanoscience and Nanotechnology, 2007

We show that asymmetric nanorods rotate under the laser radiation pressure, irrespective of the p... more We show that asymmetric nanorods rotate under the laser radiation pressure, irrespective of the polarization of the light, when trapped in laser tweezers. If a nanorod is not quite transparent to the trapping laser radiation, the radiation pressure force generates a non zero torque on the asymmetric nanorods making them rotate at a moderate speed. Our experimental observations on radiation pressure driven rotations of M gO and Si naorods in optical trap show that the efficiency of the rotors depends directly on their transmittance at the trapping wavelength. We propose theoretical models to estimate the rotational speed at different laser powers for a rotor with shape asymmetries or surface irregularities.

Irreversibility-to-reversibility crossover in transient response of an optically trapped particle

EPL (Europhysics Letters), 2011

We study the transient response of a colloidal bead which is released from different heights and ... more We study the transient response of a colloidal bead which is released from different heights and allowed to relax in the potential well of an optical trap. Depending on the initial potential energy, the system's time evolution shows dramatically different behaviors. Starting from the short-time reversible to long-time irreversible transition, a stationary reversible state with zero net dissipation can be achieved as the release point energy is decreased. If the system starts with even lower energy, it progressively extracts useful work from thermal noise and exhibits an anomalous irreversibility. In addition, we have verified the Transient Fluctuation Theorem and the Integrated Transient Fluctuation Theorem even for the non-ergodic descriptions of our system.

Out-of-equilibrium microrheology using optical tweezers to probe directional viscoelastic properties under shear

EPL (Europhysics Letters), 2010

PACS 87.80.Cc-Optical trapping PACS 83.60.Fg-Shear rate dependent viscosity PACS 87.15.N-Properti... more PACS 87.80.Cc-Optical trapping PACS 83.60.Fg-Shear rate dependent viscosity PACS 87.15.N-Properties of solutions of macromolecules PACS 87.19.rh-Fluid transport and rheology Abstract.-Many wormlike micellar systems exhibit appreciable shear thinning due to shear induced alignment. As the micelles get aligned introducing directionality in the system, the viscoelastic properties are no longer expected to be isotropic. An optical tweezers based active microrheology technique enables us to probe the out-of-equilibrium rheological properties of a wormlike micellar system simultaneously along two orthogonal directions-parallel to the applied shear, as well as perpendicular to it. While the displacements of a trapped bead-in response to active drag force carry signature of conventional shear thinning, its spontaneous position fluctuations along the perpendicular direction manifest an orthogonal shear thickening, an effect hitherto unobserved.

Optical tweezer for probing erythrocyte membrane deformability

Applied Physics Letters, 2009

We report that the average rotation speed of optically trapped crenated erythrocytes is direct si... more We report that the average rotation speed of optically trapped crenated erythrocytes is direct signature of their membrane deformability. When placed in hypertonic buffer, discocytic erythrocytes are subjected to crenation. The deformation of cells brings in chirality and asymmetry in shape that makes them rotate under the scattering force of a linearly polarized optical trap. A change in the deformability of the erythrocytes, due to any internal or environmental factor, affects the rotation speed of the trapped crenated cells. Here we show how the increment in erythrocyte membrane rigidity with adsorption of Ca ++ ions can be exhibited through this approach.

Digital Imaging of Colloidal Systems

We propose a new algorithm to identify particles and extract quantitative information e.g. pair c... more We propose a new algorithm to identify particles and extract quantitative information e.g. pair correlation function and structure factor from a colloidal image. The algorithm works for systems of spherical particles of different sizes and can be generalized to recognize non-spherical particles also. It can distinguish particles of different sizes in the case of a polydisperse system. The most noteworthy part is that the algorithm recognizes particles with cent percent accuracy even when the particles form aggregations. Here we present pair correlation functions evaluated using our algorithm for colloidal systems that under ac electric field show different phase behaviors.

Tunable Brownian Vortex at the Interface

A general kind of Brownian vortexes are demonstrated by applying an external nonconservative forc... more A general kind of Brownian vortexes are demonstrated by applying an external nonconservative force field to a colloidal particle bound by a conservative optical trapping force at a liquid-air interface. As the liquid medium is translated at a constant velocity with the bead trapped at the interface, the drag force near the surface provide enough rotational component to bias the particle's thermal fluctuations in a circulatory motion. The interplay between the thermal fluctuations and the advection of the bead in constituting the vortex motions is studied, inferring that the angular velocity of the circulatory motion offers a comparative measure of the interface fluctuations.

epl draft Irreversibility to Reversibility Crossover in Transient Response of an Optically Trapped Particle

PACS 05.70.Ln-Nonequilibrium and irreversible thermodynamics PACS 05.40.-a-Fluctuation phenomena,... more PACS 05.70.Ln-Nonequilibrium and irreversible thermodynamics PACS 05.40.-a-Fluctuation phenomena, random processes, noise, and Brownian motion PACS 42.50.Wk-Mechanical effects of light on material media, microstructures and particles Abstract.-We study the transient response of a colloidal bead which is released from different heights and allowed to relax in the potential well of an optical trap. Depending on the initial potential energy, the system's time evolution shows dramatically different behaviors. Starting from the short-time reversible to long-time irreversible transition, a stationary reversible state with zero net dissipation can be achieved as the release point energy is decreased. If the system starts with even lower energy, it progressively extracts useful work from thermal noise and exhibits an anomalous irreversibility. In addition, we have verified the Transient Fluctuation Theorem and the Integrated Transient Fluctuation Theorem even for the non-ergodic descriptions of our system.

Random-Walk Trajectories of Probe Particles in Viscoelastic Complex Fluids

Bulletin of the American Physical Society, 2014

Optical Tweezers To Probe And Manipulate Soft, Nano And Bio Systems

Random walks of colloidal probes in viscoelastic materials

Physical Review E, Apr 1, 2014

To overcome limitations of using a single fixed time step in random walk simulations, such as tho... more To overcome limitations of using a single fixed time step in random walk simulations, such as those that rely on the classic Wiener approach, we have developed an algorithm for exploring random walks based on random temporal steps that are uniformly distributed in logarithmic time. This improvement enables us to generate random-walk trajectories of probe particles that span a highly extended dynamic range in time, thereby facilitating the exploration of probe motion in soft viscoelastic materials. By combining this faster approach with a Maxwell-Voigt model (MVM) of linear viscoelasticity, based on a slowly diffusing harmonically bound Brownian particle, we rapidly create trajectories of spherical probes in soft viscoelastic materials over more than 12 orders of magnitude in time. Appropriate windowing of these trajectories over different time intervals demonstrates that random walk for the MVM is neither self-similar nor self-affine, even if the viscoelastic material is isotropic. We extend this approach to spatially anisotropic viscoelastic materials, using binning to calculate the anisotropic mean square displacements and creep compliances along different orthogonal directions. The elimination of a fixed time step in simulations of random processes, including random walks, opens up interesting possibilities for modeling dynamics and response over a highly extended temporal dynamic range.