Droplet sensors for fuel systems

A system comprising:

• a microfluidic channel configured to receive a flow of a first fluid and a second fluid dispersed in the first fluid, wherein the second fluid has a different composition than the first fluid;
• a light source configured to direct a light beam in a frequency band along a path through the microfluidic channel, wherein the frequency band is selected to have a higher absorbance by the second fluid than by the first fluid;
• an aperture element defining a light aperture positioned in the path of the light beam from the light source;
• a light detector positioned to receive the light beam in a sensing area after passing through the microfluidic channel and the light aperture, the light detector configured to provide a signal representing an amount of light in the frequency band that remains after passing through the microfluidic channel;
• and
• a controller operably coupled to the light detector and configured to determine whether the second fluid is in droplet form based on the signal,
• wherein the microfluidic channel is in parallel fluid communication with a main flow of a vehicle engine fuel line.