Microfluidic devices and systems

A method of label-free cell or particle sorting in a microfluidic device includes providing a stream of aqueous droplets in oil in a channel of the microfluidic device, wherein at least some of the droplets include cells or particles and illuminating the stream from a first direction. Then, one detects scattered light from cells or particles within said aqueous droplets in a second direction. Next, one determines a number of the cells or particles in each droplet from the scattered light and sorts the aqueous droplets into differentiated streams. Notably, the refractive index of said oil is modified to closely match a refractive index of the aqueous droplets to reduce light scattered from boundaries of the droplets. A volume of the droplets can also controlled.