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The presence of charge in non-aqueous colloidal systems is often disregarded, despite persistent reports of its existence in literature. In recent years the importance of charge in non-aqueous systems has become more apparent, with practical applications in the development of electrophoretic inks and photonic crystals.
Charges on particles in aqueous colloidal systems are on the order of thousands of electrons, whereas in non-aqueous systems they range from a few electrons to hundreds of electrons. The question is this: How to measure such small charges? Phase Analysis Light Scattering (PALS) allows us to look at the mobilities in a large sample of particles, but at very small charge densities it requires very long experiments and it is difficult to obtain robust results.
Single Particle Optical Electrophoresis is a new method to measure charge in colloidal systems. We use an electrophoretic cell in combination with an optical tweezers setup to measure the motion of a single colloidal particle in an oscillating electric field with sub-nanometre precision. This method allows us to measure very low mobilities, equivalent to charges of only a few electrons, to investigate the field and frequency dependence of the mobility and to look at the charge distribution within a system.
This experiment also allows us to give practical results showing the behaviour of a Brownian oscillator in an external, time-dependent field. We can illustrate this by looking at the autocorrelations, power spectra or PDF's (probabilty distribution functions) of the data.
[1] Leunissen, M. E., Christova, C. G. et al.
"Ionic colloidal crystals of oppositely charged particles"
Nature 437 (2005) p.235
[2] Shevchenko, E. V., Talapin, D. V. et al.
"Structural diversity in binary nanoparticle superlattices"
Nature Letters 439 (2006) p.55