Sharif Digital Repository

Optical tweezers consist of a tightly focused laser beam. A particle with refractive index greater than that of the surrounding medium can be trapped at the focus of the laser. The trapped object experiences a three-dimensional Hookean restoring force towards the focus. Nano (micron)-sized spheres can produce forces in the range of few pico-newton to few nano-newton. This range covers a large number of the forces, which contribute in biological processes; therefore, optical tweezers are very often used for micromanipulation of biological tissues. In a typical micromanipulation experiment it is crucial to perform proper positional calibration prior to use. There are several calibrate methods... 

The base of dielectrophoresis(DEP) technique has been demonstrated through electro-dynamic interactions between an applied non-uniform AC electric field and induced distributed dipoles within a polarized. In particle manipulation and/or separation using the DEP method, knowledge of precise DEP force of a particle is essential in designing a DEP device and improving its separation efficiency and accuracy. In most DEP studies related to particle manipulation and/or separation, however their absolute DEP forces remain unknown[2]. Insufficient information regarding the DEP behavior of particles can result in preventing a strategic maneuver for efficient particle manipulation and/or separation... 

Red blood cells (RBC, erythrocytes) are cells with an average diameter of 8 microns, the thickness of 2-3 microns and the volume of 90 fl. The biconcave shape of red blood cells,together with cell membrane flexibility, enables them to pass the capillaries whose diameter is considerably less than 8 micrometers. Mechanical properties of RBC are related to their membrane and cytoskeleton, which consists of spectrin network. Optical tweezers is able to exert very precise forces to erythrocytes using beed as a handle. In this report, we studied mechanical properties of RBC such as elasticity and viscoelasticity using single trap optical tweezers with beads with antibody coated in order to attach... 

Blood is one of the most important biological organs. Red blood cell (RBC), as the most abundant cell of blood, plays an important role at life of the living bodies. Unique mechanical properties of RBCs enable them to pass through very narrow vessels. Any abnormality in these mechanical properties will disorder the function of the cell and many diseases affect these properties too. Studying the mechanical properties of RBCs is very importantin that it could result in a disease diagnostic method in a single-cell level. In this thesis we have tried to measure the viscoelastic properties of a single RBC by an optical trap through a passive method. This measurement is made through studying the... 

Red blood cells are one of the most important parts of blood and their mechanical properties are so important in some diseases. Red blood cell is mostly consisted of membrane, cell skeleton, and cytoplasm which contains hemoglobin proteins. Studying these properties separately not only could give us pieces of information about cell mechanics, but also reveals a way to predict their behavior after illnesses. Optical tweezers as a tool, which can produce and measure Piconewton to Nanonewton forces, has been interesting in red blood cell parts' studying. In this thesis, at first hemoglobin is taken out gradually by using Saponin (which is enable to make some pores in the membrane) and the... 

Viscoelastic properties of biological fluid play an important role on biological organs. As an example we all know that Red blood cells must have high flexibility to pass thorough thin capillary.This high flexibility come partially from cytoplasm and 2 dimensional membrane and partially from polymeric fluid inside the cell(hemoglobin). So, for recognizing mechanical properties of a whole cell we also need to study the same properties just for fluid inside the cell. for this general purpose through this project we are going to measure viscoelastic properties of any polymeric fluid by optical tweezers which has an ability for measuring displacement of micron particle with accuracy of nm. first... 

Intensity or phase Modulation of a wavefront is possible by means of optical devices. Placing a grating or lens on the way of a wavefront, will alter the phase or intensity of it and this will make a diffraction pattern. Spatial Light Modulator (SLM) is an optical device which is the most important part of a video projector. One can apply grating, lens and other phases on the wavefront using this device.In this project we used SLM to modify phase of laser beam and then we made many focal points such that they could displace independently. The result of this project can be used in designing and creating a holographic optical tweezers.Moreover, A. Einstein proved that the light could be... 

The electromagnetic (EM) force is the pivot on which this thesis revolves. We inspect different formulations of the distribution of the EM force (and momentum) inside matter, and show that the Einstein-Laub force density is incompatible with special relativity. We study the exerted EM force on small particles in depth, and explain when the conventional dipole approximation fails to yield accurate results. We propose an all-dielectric structure which is able to trap very small dielectric particles (of diameters as small as 10 nm). One of the achievements of this thesis is an explanation for an adverse phenomenon observed in many experiments on the optomechanical systems employing... 

In the presence of spatially non-uniform electric field, a dielectric object can experience a translational force which is known as Dielectrophoresis (DEP) force. DEP is one of the most applicable methods in sorting mechanism of different biological cells. Moreover accurately measuring the DEP force in single cell level can lead to a valuable knwoledge about the electrical properties of various cells.In this thesis, first, the other electrokinetic forces in which the Electro-Osmosis (EO) force is the important one, have to be eliminated. Regarding that this effect is strongly dependent on the frequency of external E field, we have applied the external electric field at appropriate frequency... 

Microfluidics is a field of science that studies micron-scale characteristics of fluids. In this scale fluids show fascinating behavior far beyond our expectation compared to their large-scale counterparts. One of such peculiar behavior is the mixing of two different liquids flowing inside a microchannel. Due to low Reynolds number, the flow inside such a channel would be dominated by laminar behavior. In this regime mixing of the fluids is only mediated by molecular diffusion which is a rather slow process. In the current thesis, we utilized the rotating ability of optical tweezers to construct an active micromixer for Microfluidics. In order to do this we used birefringent microbeads. The... 

Optical Tweezers (OT) are non-invasive micro-manipulation tools based on a tightly focused Gaussian laser beam for applying and measuring pico-Newton range optical forces. They are also utilized for trapping and moving particles and biological specimens. The force exerted by OT is mainly determined by detecting the position of the trapped object inside the harmonic potential well of OT. Conventionally,a Quadrant Photo-Diode (QPD) positioned at the Back Focal Plane (BFP) of the condenser lens provides the position of the bead with MHz temporal and nanometer spatial resolution. As a disadvantage of this detection scheme, it is impossible to obtain simultaneously the maximal lateral and axial... 

Properties of viscoelastic of biopolymers have important role in cells structure; for example: cell motility depends on viscoelastic propertie of Actin polymer. therefore according to its importance ,in this thesis we intend to examine the viscoelastic modulus using Optical tweezers and dissipation theorem.this method is benefits than methods of active microrheology. for example this method can be used for low concentrations; because in the low concentrations, the exerting force will cause changing in the state of matter that here we investigate destructive force effect. we investigate viscoelastic of water for method accuracy then obtain viscoelastic of Kelcogel  

Blood is one of the most important fluids in the human body, consisting of three parts: red blood cells, white blood cells and platelets. The mechanical properties of red blood cells are important in many diseases. The study of viscosity and other mechanical properties of red blood cells in recent years has attracted the attention of scientists in this field. The mechanical properties of red blood cells are related to the outer membrane and its cytoskeleton, and the study of these components can predict the behavior of red blood cells in diseases. The thyroid gland is located in front of the neck and secretes hormones that control many chemical processes in different parts of the body.... 

In this thesis, the optical force acting on the spherical polystyrene particle trapped by optical tweezers is calculated. The calculation is based on the generalized Lorenz-Mie theory. The effect of various parameters on the optical force is also investigated. It is shown that, tuning the different parameters, one can significantly enhance the optical force acting on the trapped particle. Besides, one can achieve a stable 3D trap of a sub-micron particle somewhere far deep inside the sample chamber. Our results indicate that: (1) By obstructing the central region of the incident Gaussian laser beam at the back focal plane of the objective lens, the trap stiffness, the maximum linear force,... 

Reconstruction of stochastic dynamical equations from measured timeseries of complex systems plays a crucial role in retrieving dynamic information and studying macroscopic states of such systems. This method which has been pursued for decades is practical in diverse fields like physics, biology, ecology, economics, social science, neuroscience and so on. To obtain stochastic dynamic equation, we have to calculate Kramers-Moyal coefficients from Markovian processes measured time series. We aim to propose new method for deriving these coefficients which is more accurate than two current methods, namely the histogram and kernel methods. In this dissertation, we studied dynamic equations of... 

Optical tweezers consist of a tightly focused laser beam. Particle with refractive index grater than that of the surrounding medium are eligible for optical trapping. This device is used for measuring forces from pico-newton to nano-newton, and displacements from nanometer to micrometer. In biopolymers, for example, it is used for studying the elasticity of DNA molecules, mechanical properties of cell membrane; and also in Microfluidics, it can is used for mixing two liquids flowing in a micro-channel by rotating a trapped particle. In general, physics of optical tweezers (or interaction of the trapped particle with the tightly focused laser beam) is treated in three different regimes:1)...