Droplet microfluidics enables high-throughput, compartmentalized reactions using minimal reagent volumes, but most implementations rely on precision-fabricated chips and external pumping systems that limit portability and accessibility. Here, we present a handheld vibrational droplet generator that repurposes a consumer electric toothbrush and a modified disposable pipette tip to produce nearly monodisperse water-in-oil droplets without microfluidic channels or syringe pumps. The device is powered by the toothbrush’s built-in motor and controlled by a simple 3D-printed adapter and adjustable counterweight that tune the vibration amplitude transmitted to the pipette tip. By varying the aperture of the pipette tip, droplets with diameters from ~100–300 µm were generated at rates of ~100 droplets s⁻¹. Image analysis revealed narrow size distributions with coefficients of variation below 5% in typical operating conditions. We further demonstrate proof-of-concept applications in digital loop-mediated isothermal amplification (LAMP) and microbiological colony-forming unit (CFU) assays. A commercial feline parvovirus (FPV) kit manufactured by Beyotime Biotechnology Co., Ltd. (Shanghai, China), three template concentrations yielded emulsified reaction droplets that remained stable at 65 °C for 45 min and produced distinct fractions of fluorescent-positive droplets, allowing estimation of template concentration via a Poisson model. In a second set of experiments, the device was used as a droplet-based spreader to dispense diluted Escherichia coli suspensions onto LB agar plates, achieving uniform colony distributions across the plate at different dilution factors. The proposed handheld vibrational generator is inexpensive, easy to assemble from off-the-shelf components, and minimizes dead volume and cross-contamination because only the pipette tip contacts the sample. Although the current prototype still exhibits device-to-device variability and moving droplets in open containers complicate real-time imaging, these results indicate that toothbrush-based vibrational actuation can provide a practical and scalable route toward “lab-in-hand” droplet assays in resource-limited or educational settings.

Droplet microarrays are increasingly used for miniaturized, high-throughput biochemical assays, yet their fabrication commonly relies on complex lithographic processes, custom masks, or specialized coatings. Here we present a simple method for generating hydrophilic arrays on hydrophobic plastic substrates by combining ultrasonic atomization with off-the-shelf perforated masks. A fine mist of poly(vinyl alcohol) (PVA) solution is directed through commercial diamond sieves onto polypropylene (PP) sheets and polystyrene (PS) sheets, forming hydrophilic spots surrounded by the native hydrophobic background. Static contact angle measurements confirm a strong local contrast in wettability (from 100.85 ± 0.91° on untreated PP to 39.96 ± 0.71° on patterned spots, from 95.68 ± 3.61° on untreated PS to 52.00 ± 0.85° on patterned spots), while Image analysis shows droplet CVs of 6–8% in aqueous dye solutions for 1.2–2.0 mm masks; in complex media (LB), droplet uniformity decreases. By mounting the moving mask on a motorized stage, we generate one-dimensional reagent gradients simply by controlling the moving mask motion during atomization. We further demonstrate biological compatibility by culturing Escherichia coli in LB droplets containing resazurin, and by performing localized antibiotic screening using a moving mask-guided streptomycin gradient. The resulting droplet-wise viability data yield an on-chip dose–response curve with an IC₅₀ of 5.1 µg · mL⁻¹ (95% CI: 4.5–5.6 µg·mL⁻¹), obtained from a single array. Covering droplets with Electronic Fluorinated Fluid maintains volumes within 5% of their initial value over 24 h. Compared with conventional droplet microarray fabrication, the proposed method eliminates custom mask production and cleanroom steps, is compatible with standard plastic labware, and intrinsically supports spatial gradients. These attributes make masked ultrasonic atomization a practical platform for high-throughput microfluidic assays, especially in resource-limited settings.