Fig. 2: Evaporation steps.

a Spreading: the droplet spreads within and above the membrane and pushes particles towards the hydrophobic barrier. b Fixed contact radius: as evaporation begins, an internal flow develops within the residual droplet, driving particles toward the edge, forming a coffee-ring pattern. c Fixed contact angle: once the contract angle drops below a certain value (~ 5°), the droplet starts to shrink while maintaining this fixed angle, pulling the remaining particles towards the center. d Backward evaporation: The residual solution within the membrane evaporates inward, with minimal influence on the overall particle deposition pattern. Insets: particle counts where the coffee-ring positions by dashed blue lines, initial droplet boundaries by dashed red lines, and center of the residual droplet by dashed blue line plus C as center. Note, these schematic patterns are based on both our experimental data and theoretical studies^52,53,54,55. e The sample droplet deposition pattern: (i) confocal microscopy image of the deposition of the sample droplet by using the fluorescent dyes. The high dye concentrations are at the spreading boundary and the coffee-ring of the residual droplet as samples are accumulated in these two places. (ii) Normalized grey index value versus axial distance of the central line passing across the image. (f) The GNShs droplet deposition pattern. (i) Grayscale image shows different areas on the detection zone, including residual sample droplet (yellow), sample spreading zone (green), GNShs residual drop (red), and GNShs spreading zone (blue). The asymmetric pattern at the overlapping area shows the gradual grey index value changes as the plasmonic residual droplet is pushing and interacting with the high concentration samples at the coffee ring of the residual sample droplet. (ii) Normalized grey index value for a central line passing through the plasmonic droplet coffee-ring showing the non-uniformity and the gradient in the overlapping area. Data are reported as mean values ± standard deviations (SD), based on n = 3 sample lines. It is noted that high gray index values imply less particles. These measurements are from the N-Protein at 1000 ng/ml concentration. Note that all experiments were repeated at least three times.