Measuring the size of oil droplets in a flow cytometer using Mie resonances: A possible size calibration ladder for 0.5-6 μm

An emulsion of silicone oil droplets in aqueous buffer produces a distinctive series of peaks or resonances in the side scatter histogram in a flow cytometer. As many as 12 peaks are observed in the violet-side scatter channel at 405 nm, with half that number observed in the blue side scatter channel at 488 nm. Using the index of refraction of the oil and buffer, the wavelength of light, and the collection angle and gain of the instrument, we assign the peaks to specific diameters at which Mie resonances occur. With the close match for the index of refraction of silicone oil (n = 1.417 at 405 nm) to biological materials, these resonances could form the basis of a finely spaced size calibration ladder in the range 0.5-6 μm for estimating the size of biological particles in a flow cytometer. Resonances were also observed using mineral oil (n = 1.483 at 405 nm) suggesting that investigating and modeling resonances for emulsion systems may be useful for understanding these systems.