Question

How does optical tweezer relate to force and flow?

please i need specific answer for each part not from google and no handwritten.

Answer 1

Tweezers technology enables the quantification of molecular, cellular and micro-logical processes. Applications include molecular motor mechanics, binding/elasticity of DNA and proteins, cell membrane dynamics and particle uptake into cells. The scientist is used in material science and soft matter applications ranging from the characterization of mechanical properties such as adhesion, viscous elasticity or deformation, to optical, thermodynamic or nano manipulation experiments with trapped particles.

In a typical optical tweezers configuration the incoming light originates from a focused laser beam through a microscope objective and focuses on a spot in the sample. This scattering produces a net momentum transfer from the light photons to the object and causes the bead to be pushed towards the beam propagation.

Scattering, in physics, a change in the direction of motion of a particle because of a collision with another particle. As defined in physics, a collision can occur between particles that repel one another, such as two positive (or negative) ions, and need not involve direct physical contact of the particles. Experiments with subatomic particles indicate that the electric repulsive force between the particles satisfies Coulomb’s law, which states that the force varies as the inverse square of the distance between the particles; i.e., if the distance is halved, the force is quadrupled. Experiments show, that the trajectory of the scattered particle, whatever the angle of deflection, is a hyperbola and that as the bombarding particle is aimed more closely toward the scattering center the angle of deflection increases.

The correct physical description of optical trapping depends on the size of the trapped object. One speaks of the ‘ray optics’ regime when the object’s dimension d is much larger than the wavelength of the trapping light: d>>lambda. In this case,diffraction effects can be neglected and the trapping forces of the light can be understood in terms of ray optics. The regime where d<<. is called the Rayleigh regime. In this case, the trapped particles can be treated as point dipoles,as the electromagnetic field is constant on the scale of the particle. Obviously, if the laser is not focused, the particle will be propelled away due to the forward radiation pressure caused by light scattering.

In the ray-optics regime, the origin of the trapping force can be intuitively understood in terms of refraction of light rays between media with different indices of refraction.Qualitatively depicts the origin of the trapping forces in this regime. The lateral gradient restoring force can be understood as follows: if rays p 1 and p 2 have different intensity, the momentum changes of these rays (.p 1 and p 2, respectively) differ in magnitude, causing a net reaction force on the refracting medium in the direction of highest intensity. The x–projection of this force .p x tends to counteract a displacement from the laser beam axis, pulling the particle to the center of beam.The axial gradient force is similarly caused by momentum transfer upon refraction, resulting in a restoring force towards the focus. The scattering force would cause the object to be propelled out of the focus (along the positive z–direction). The object is stably trapped only if the scattering force along the positive z–direction is compensated by the gradient force along the negative one. To achieve this, a tight focus is needed, with a significant fraction of the incident light coming in from large angles.

upon the trapped object. These external forces tend to push or pull the object from the center of the trap. The refractive object, in turn, acts like a little lens and refracts the rays passing through it. The far-field interference of the laser light with the scattered light from the trapped particle , made it parallel by a detection lens, can be used to get a sensitive measure for the displacements of the trapped particle from the focus. If the trap steering scheme is well chosen, the intensity distribution in the back focal plane.It is hard or even impossible to theoretically predict the (B F P) of the condenser lens does not change when moving force exerted by the trapping laser beam from first the optical trap around in the sample; the distribution is only
principles. However, it is possible to use the trapping light affected by motion of the trapped object with respect to the that is scattered by the object to get an accurate measure laser focus. When imaging the light distribution in the B F P for external forces (i.e., other than the trapping force) acting onto a position-sensitive detector, such as a quadrant photo diode a light sensitive diode which is divided into four equal segments), displacements of the particle in the trap can be measured.