Joseph Junio

Direct measurements of the frequency-dependent dielectrophoresis force

Dielectrophoresis (DEP), the phenomenon of directed motion of electrically polarizable particles in a nonuniform electric field, is promising for applications in biochemical separation and filtration. For colloidal particles in suspension, the relaxation of the ionic species in the shear layer gives rise to a frequency-dependent, bidirectional DEP force in the radio frequency range. However, quantification methods of the DEP force on individual particles with the pico-Newton resolution required for the development of theories and design of device applications are lacking. We report the use of optical tweezers as a force sensor and a lock-in phase-sensitive technique for analysis of the particle motion in an amplitude modulated DEP force. The coherent detection and sensing scheme yielded not only unprecedented sensitivity for DEP force measurements, but also provided a selectivity that clearly distinguishes the pure DEP force from all the other forces in the system, including electrophoresis, electro-osmosis, heat-induced convection, and Brownian forces, all of which can hamper accurate measurements through other existing methods. Using optical tweezers-based force transducers already developed in our laboratory, we have results that quantify the frequency-dependent DEP force and the crossover frequency of individual particles with this new experimental method.

Ensemble method to measure the potential energy of nanoparticles in an optical trap

A method is described for measuring the potential energy of nanoparticles in an optical trap by trapping an ensemble of particles with a focused laser beam. The force balance between repulsive osmotic and confining gradient-force pressures determines the single-particle trapping potential independent of interactions between the particles. The ensemble nature of the measurement permits evaluation of single-particle trapping energies much smaller than kBT. Energies obtained by this method are compared to those of single-particle methods as well as to theoretical calculations based on classical electromagnetic optics.

Measurements of the compressibility of colloidal suspensions by radiation pressure

This paper reports an experimental study of the interparticle interactions present in a model colloid system composed of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. By independently measuring the fluorescence intensity as a function of particle number density, we were able to determine the relationship between the radiation pressure generated by the optical trap and the resulting number density increase, yielding the calculation of the isothermal compressibility of the colloid system. Optical trapping was made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linearly with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio required to detect the changes in local particle density induced by optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from the diffraction-limited focal region of the two laser beams.

Measurements of charged colloidal bulk moduli using optical bottles

We report a novel method, the optical bottle that was used to directly measure the osmotic bulk modulus for a colloid suspension. We determined the bulk modulus by optically trapping multiple nanoparticles and considered a mechanical balance between the compressive laser gradient force pressure and the resulting resistive osmotic pressure. Osmotic bulk moduli results measured with the optical bottle are presented for aqueous suspensions of latex particles as a function of solution ionic strength; and are compared to results from identical samples measured using turbidity spectra.

Depletion-enhanced optical trapping in nanoparticle suspensions

With the right proportions, a binary suspension of different sized particles may be subject to entropic effects that can generate a depletion-induced attraction between large particles. A manifestation of the induced attraction is the enhanced osmotic compressibility of the larger species in the presence of the smaller species. We conducted an experimental study on how such an enhancement is affected for 190 nm polystyrene spheres in the presence of polyethelyne-oxide in aqueous solutions. Using the gradient force from a tightly focused laser, we can locally concentrate the polystyrene nanoparticles in suspension, and from the changes of local particle density under the known gradient force, we deduce a quantitative measure of the isothermal compressibility of the particles. We report the analysis of these compressibilities and their enhancement by the added polymers for a broad range of particle and polymer concentrations.

Osmotic Bulk Modulus of Charged Colloids Measured by Ensemble Optical Trapping.

The optical-bottle technique is used to measure osmotic bulk moduli of colloid suspensions. The bulk modulus is determined by optically trapping an ensemble of nanoparticles and invoking a steady-state force balance between confining optical-gradient forces and repulsive osmotic-pressure forces. Osmotic bulk moduli are reported for aqueous suspensions of charged polystyrene particles in NaCl solutions as a function of particle concentration and ionic strength, and are compared to those determined by turbidity measurements under the same conditions. Effective particle charges are calculated from the bulk moduli and are found to increase as a function of ionic strength, consistent with previously reported results.

Fluorescence correlation spectroscopy in an optical trap

We have combined optical trapping and fluorescence correlation spectroscopy (FCS) to determine the trapping energy and concentration of nanoparticles in suspension by analyzing the elongated dwell time and enhanced concentration in the optical trap.

The Kerr effect produced by optical trapping of nanoparticles in aqueous suspensions

This paper reports an experimental study of the low laser intensity Kerr Effect produced by optical trapping of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. Optical trapping was made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linear with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio required to detect the very minute (sub-thermal fluctuation) changes in local particle density induced by optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from the diffraction-limited focal region of the two laser beams. By independently measuring the fluorescence intensity as a function of particle concentration and dn/dC (the change in refractive index due to change in concentration), we were able to determine the Kerr coefficients for laser trap powers in the range of 10.6 mW to 85 mW. Non-linear behavior in the refractive index vs. laser intensity relationship indicates that higher order Kerr coefficients are needed to describe the Kerr effect. Kerr coefficients obtained by using a circularly polarized IR laser were similar to those obtained by a linearly polarized laser, indicating that the induced electric dipole-dipole interactions did not contribute to the electric field-induced concentration changes giving rise to the Kerr effect.