Optical tweezers is the most convenient and widespread technique for spatial localization and manipulation of particles with dimensions of a few microns or less. To date, optical tweezers have developed into a powerful tool in atom optics, statistical physics, molecular biology, biochemistry, and biophysics.

The principle of optical manipulation is based on the fact that in a non-uniform electromagnetic field, particles with a positive polarisability tend to be localized in regions of maximum field energy density. In optics, it means that particles with a refractive index higher than that of the surrounding medium experience a force directed towards the intensity maximum. In the field of a focused light beam, the intensity is maximal in the waist of the beam and the particles are localized in it. The manipulation of particles can therefore be realized by changing the focal point.

Optical tweezers enable not only the manipulation of nano- and microparticles, but also measurements of forces acting on the microscale. When an external force acts on the trapped particle, its equilibrium position changes so that the trapping force compensates the external one. Its value, therefore, can be obtained as a result of determining the spatial position of the particle inside the trap. The mapping technique based on this principle is known as photonic force microscopy.

RBC aggregation studied by double trap optical tweezers

We use double trap optical tweezers for the direct measurements of aggregation forces in piconewton range between red blood cells (RBCs) in pair rouleau under physiological conditions.

We are also involved into the RBCs elastic properties research offering a novel method for precise monitoring of red blood cells viscoelastic properties using active rheology approach combined with the forced RBC edges vibration analysis.

Magnetic microbeads interaction studied in optical tweezers

Magnetic liquids, which are suspensions of magnetic microparticles, have been actively studied in recent decades. These suspensions have numerous applications starting from the production of magnetic memory devices to cancer treatment by the hyperthermia method. Pair magnetic interaction forces on the micro-scales are as small as hundreds of fN, but play an important role in determination of magnetic fluids properties. Optical tweezers technique is a unique approach to study these magnetic forces.

Measurement of interaction force between two individual microparticles
In experiments two individual magnetic microbeads are localized in two independent optical traps. Magnetic moments are induced in the microbeads in presence of external magnetic field and microbeads magnetically interact with each other. Magnetic interaction leads to the displacement of the microbeads from the optical trap centers. In this equilibrium position interaction force is equal to optical trap restoring force, which allows measuring the value of magnetic interaction force directly, since the optical trapping force is known. Interaction force depends on the microparticle magnetic moments, so there is an ability to obtain the magnetic moment values for the individual microparticles with precision of fAm² .

Brownian motion of two magnetic microparticles
Microbeads in optical tweezers are dipped in liquid and undergo Brownian motion. Magnetic interaction makes the motion of microparticles more coupled, more correlated. It was observed that the cross-correlation functions of microbeads Brownian displacements considerably change in the presence of an external magnetic field. The changes in the cross-correlation functions are dependent on the values and directions of magnetic interaction forces. In case of attraction force between particles - the cross-correlation function values decrease, which means that the motion of particles becomes anti-correlated and the particles move in opposite directions. When repulsive force acts between particles - the cross-correlation values increase. The particles move in the same directions and their motion becomes correlated. Such correlations in particle motion considerably affect various processes in magnetic suspensions, such as magnetic particle aggregation, magnetic fluid flow, and magnetic field induced structure formation.

Synchronization in motion of two magnetically bounded oscillators - active microrheology approach
Effect of correlations in motion magnetic microbeads can be also detected using active microrheology method. In this case the position of one of the optical traps was oscillating and the trapped magnetic particle moved harmonically. Magnetic interaction force acting on the second particle depended on the distance between particles, so this force was harmonically modulated, forcing the second particle to move periodically too. When particles are close to each other, their motion is strongly synchronized, and this synhronization can be seen with the naked eye.