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 µm 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 °C, more of the larger spheres (150–200 µm) are deposited near the droplet center, but smaller spheres <50 µm 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 °C, higher than the boiling point of IPA, most of the spheres <50 µm are transported close to the droplet edge. In the center of the dried pattern obtained from a 0.5 µl droplet, the spheres with >150 µm diameter outnumber those with <50 µm diameter by 6×. The deposited spheres remain attached to the substrate surface when dry. The self-assembled nature of this drying pattern results in size sorting.

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Cenospheres are lightweight, inert hollow spheres which are useful in a large number of applications, particularly as fillers. This paper describes the mechanical resonant characteristics of miniature hollow hemispheres fabricated from sodium borosilicate cenospheres for potential use as sensing elements in inertial sensors, biochemical detectors and other devices. The tested structures are approximately 200 μm, 150 μm and 1.8 μm in diameter, height and thickness, respectively, with the dome anchored and the rim free. The lowest order wine glass modes of these structures are experimentally determined (and confirmed by modeling) to be in the range of 300-600 kHz-in the range of those of planar micro-resonators that have been explored for sensing applications.

Marangoni flow, the flow of liquids induced by a temperature gradient at a liquid-gas interface, can be utilized in microfluidic actuation. In this paper, we describe the rotary motion, by Marangoni flow, of a micromotor made of stainless steel immersed in a thin layer of liquid. A 128-pixel heater array is suspended 500 m above the liquid of height 2 mm. By applying 1 W to each of the four heaters directly above the four rotor blades, a temperature gradient of 0.8 K/mm is created at the surface. A rotor of diameter 4.1 mm with a blade angle of 34° experiences a maximum torque of 5.6 pN.m and takes 28 sec to make a complete rotation in a liquid with viscosity 5 cSt. The speed of rotation is affected by the blade angle of the rotor, temperature gradient of liquid surface and liquid viscosity.