A microfluidic device comprises upper and lower spaced apart substrates defining a fluid chamber therebetween; an aperture for introducing fluid into the fluid chamber; a plurality of independently addressable array elements, each array element defining a respective region of the fluid chamber; and control means for addressing the array elements. The control means are configured to: determine that a working fluid has been introduced into a first region of the fluid chamber; and provide an output to a user to indicate that the working fluid is present in the first region.
Once the working fluid is in the first region, the fluid applicator used to dispense the fluid can be removed without any risk of accidentally withdrawing dispensed working fluid from the microfluidic device. In the case of manual loading of the working fluid the output may inform a user that it is safe to remove the applicator, or in the case of automatic or robotic loading the output signal may be provided to the system controlling the automatic or robotic loading of fluid so that the system can remove the fluid applicator.