Fig. 4: Collective actuation of microdroplets for droplet collection and biomedical analysis.

a A field modulation layer with a lattice pattern was used to guide the microdroplets coalescence. For a square lattice pattern, b the calculated magnetic flux density (horizontal projection) on the slippery surface also followed a square lattice pattern. c When micro-sized water droplets were sprayed on the surface, they coalesced and formed a square lattice pattern. d, e For a modulation layer with a N-shape lattice pattern, the calculated magnetic flux density (horizontal projection) shown in d and the coalesced microdroplets shown in e also followed a N-shape lattice pattern. Dashed lines in e are eye-guide. f–h This property can be used to collect respiratory droplets and detect the pathogens. f For a proof-of-concept demonstration, microdroplets containing dyed E. coli were sprayed on the surface with a square lattice pattern field modulation layer. g Without magnetic actuation, some microdroplets randomly coalesce, and showed fluorescence signals that were randomly distributed and were barely above noise level (left image). With magnetic actuation, all the microdroplets coalesce to form a lattice pattern, showing strong and regularly distributed fluorescence signals (right image). The bottom graph shows the fluorescence intensity along the dashed oranges lines on the images. h The fluorescence intensity increases almost linearly with the logarithm of the E. coli concentration, showing potential for quantification. Error bars represent the standard deviation (n = 3).