Cytoskeletal microtubules have been proposed to influence cell shape and mechanics based on their ability to resist large-scale compressive forces exerted by the surrounding contractile cytoskeleton. Consistent with this, cytoplasmic microtubules are often highly curved and appear buckled because of compressive loads. However, the results of in vitro studies suggest that microtubules should buckle at much larger length scales, withstanding only exceedingly small compressive forces. This discrepancy calls into question the structural role of microtubules, and highlights our lack of quantitative knowledge of the magnitude of the forces they experience and can withstand in living cells. We show that intracellular microtubules do bear large-scale compressive loads from a variety of physiological forces, but their buckling wavelength is reduced significantly because of mechanical coupling to the surrounding elastic cytoskeleton. We quantitatively explain this behavior, and show that this coupling dramatically increases the compressive forces that microtubules can sustain, suggesting they can make a more significant structural contribution to the mechanical behavior of the cell than previously thought possible.

We demonstrate a high-throughput drop sorter for microfluidic devices that uses dielectrophoretic forces. Microelectrodes underneath a polydimethylsiloxane channel produce forces of more than 10 nN on a water drop in an inert oil, resulting in sorting rates greater than 1.6 kHz. We investigate the dependence of such forces on drop size and flow. Alternate designs with electrodes on either side of a symmetric channel Y junction provide refined control over droplet selection.

The use of microfluidic devices to control drops of water in a carrier oil is a promising means of performing biological and chemical assays. An essential requirement for this is the controlled coalescence of pairs of drops to mix reagents together. We show that this can be accomplished through electrocoalescence of drops synchronized by size-dependent flow in microfluidic channels. Smaller drops move faster due to the Poiseuille flow, allowing pairs of surfactant-stabilized drops to be brought into contact where they are coalesced with an electric field. We apply this method to an enzyme assay to measure enzyme kinetic constants.