We generate droplets in a microfluidic device using a traveling surface acoustic wave (TSAW), and control droplet size by adjusting TSAW power and duration. We combine droplet production and fluorescence detection to selectively-encapsulate cells and beads; with this active method, the overwhelming majority of single particles or cells are encapsulated individually into droplets, contrasting the Poisson distribution of encapsulation number that governs traditional, passive microfluidic encapsulation. In addition, we lyse cells before selective encapsulation, and pico-inject new materials into existing droplets.