Size sorting of floating spheres based on Marangoni forces in evaporating droplets

The high throughput size sorting of particles in liquid suspensions is of interest for a variety of microanalytical and micromanufacturing applications. Hollow glass cenospheres of various diameters ranging from 5 to 200 mu m are sorted according to size by evaporation of isopropyl alcohol droplets on an unpatterned glass substrate. By raising the temperature of the glass substrate, a stable Marangoni convection is developed inside the droplet. At a substrate temperature of 55 degrees C, more of the larger spheres (150-200 mu m) are deposited near the droplet center, but smaller spheres < 50 mu m are found everywhere throughout the dried region. Better sorting is observed when the temperature of the substrate is above the boiling point of the liquid. When the substrate temperature is 85 degrees C, higher than the boiling point of IPA, most of the spheres < 50 mu m are transported close to the droplet edge. In the center of the dried pattern obtained from a 0.5 mu l droplet, the spheres with > 150 mu m diameter outnumber those with < 50 mu m diameter by 6x. The deposited spheres remain attached to the substrate surface when dry. The self-assembled nature of this drying pattern results in size sorting.