The invention relates to a microfluidic method for measuring viscosity in a micro droplet in a microfluidic system, comprising the steps of i) introducing a fluorescent molecule into a micro droplet otherwise comprising a fluid, ii) in the microfluidic system, exciting the fluorescent molecule in said micro droplet by applying light to the micro droplet, iii) measuring the resulting fluorescence emitted from the micro droplet thereby determining the viscosity of the fluid in the micro droplet. The invention also relates to method of screening for microorganisms or cells that produce viscosity- modulating compounds with desired properties. Finally, the invention also relates to the use of fluorescent molecules for measuring the viscosity of a fluid in a micro droplet in a microfluidic system.
