Our device's mixing capabilities have been evaluated using normalised grey-scale analysis, with a standard deviation threshold of 10 % denoting sufficient mixing. Mixing has been performed using a constant excitation frequency of 69 kHz. Steady volume flows of 0.3 µl min -1 and 0.66 µl min -1 for de-ionised water and black ink (4001, Pelikan) were achieved by syringe pumps (neMESYS, Cetoni ).