Optical manipulation of charged microparticles in polar fluids

Abstract

In this study, we report a systematic study of the response of a charged microparticle confined in an optical trap and driven by electric fields. The particle is embedded in a polar fluid, hence, the role of ions and counterions forming a double layer around the electrodes and the particle surface itself has been taken into account. We analyze two different cases: (i) electrodes energized by a step-wise voltage (DC mode) and (ii) electrodes driven by a sinusoidal voltage (AC mode). The experimental outcomes are analyzed in terms of a model that combines the electric response of the electrolytic cell and the motion of the trapped particle. In particular, for the DC mode we analyze the transient particle motion and correlate it with the electric current flowing in the cell. For the AC mode, the stochastic and deterministic motion of the trapped particle is analyzed either in the frequency domain (power spectral density, PSD) or in the time domain (autocorrelation function). Moreover, we will show how these different approaches (DC and AC modes) allow us, assuming predictable the applied electric field (here generated by plane parallel electrodes), to provide accurate estimation (3%) of the net charge carried by the microparticle. Vice versa, we also demonstrate how, once predetermined the charge, the trapped particle acts as a sensitive probe to reveal locally electric fields generated by arbitrary electrode geometries (in this work, wire-tip geometry).

Surface charge and hydrodynamic coefficient measurements of Bacillus subtilis spore by optical tweezers

“…Nevertheless after a certain value of the frequency the response starts to decrease because the particle cannot follow the oscillating driving field due to the viscosity of medium (the viscous medium acts like a low pass filter for the particle motion). In this experiment, we chose a modulation frequency f d = 96.7 Hz which optimize the response of the trapped particle to the external field (for more details see [26]). A typical autocorrelation function for the three axis is shown in Fig.2A, it is clearly visible the high signal to noise ratio achievable and that the oscillation of the particle occurs only along the z-axis where the uniform electric field was applied.…”

Simultaneous measurements of electrophoretic and dielectrophoretic forces using optical tweezers

“…We measured the electric charge of a trapped bead applying the external voltage only at the parallel ITO electrodes as described in [25]. In this case, the electric field was uniform (B = 0 in Eq.…”

“…We found the modulation frequency f m = 86.7 Hz to optimize the motion amplitude condition (for more details, see ref. [25,26]). The bead's effective charge value resulted to be Q = (−1.63 ± 0.05) × 10 −16 C. At the end of the experiment, we measured again the charge and verified that its value was unchanged.…”

“…* giuseppe. pesce@fisica.unina.it Recently, several experiments demonstrated the possibility to combine photonic and EP forces for estimating the electric charge carried by a single polystyrene microsphere [21][22][23][24][25][26], or even for mapping the electric field generated in simple electrodes' geometries [27]. In the present work, we discuss a new approach to determine simultaneously EP and DEP forces in proximity of microelectrodes.…”

Optical Forces at the Nanoscale: Size and Electrostatic Effects

“…In this sense we have to point out that, as it has been already published, the zeta potential values estimated from conventional bulk zeta potential measurements and those obtained from direct single particle measurements are equivalent. 30 In addition, for each particle and experimental condition, optical forces were systematically measured over different nanoparticles to perform statistical analysis. The uncertainty of OF measurements were calculated in each case and included as error bas in Figure 5.…”