Given the view of the Standard Model where the electron and quarks are just completely separate matter fields, there is at first sight no clear theoretical requirement for that.
Clear experiment diagram which explains that the droplet mass determined with Stoke's law:
Video 1. Quantum Mechanics 4a - Atoms I by ViaScience (2013) Source.
Here's some actual footage of a droplet on a well described more one-off setup:
Video 2. Millikan's Experiment, Part 2: The Experiment by Phil Furneaux (2017) Source. From Lancaster University