Motion of droplets on surfaces

Dropwise condensation on radial energy gradients	Settlement of algal spores on radial energy gradients

Migration of a liquid drop induced by a linear surface energy gradient

Transport of liquid drops around a chip using patterned surface energy gradients

             Not all processes when scaled down to the microscale are best suited by the usual continuous flow paradigms of typical labs-on-a-chip. For example, many biological processes are typically batchwise at the lab scale. An alternative strategy is to transport discrete droplets of liquid around a chip to various unit operations. The Daniel group employs surfaces using gradients in adhesion to propel drops by a Marangoni force (surface tension gradient derived from chemical heterogeneity patterned on the surface) or motion induced by vibration of the support. While working in Professor Manoj Chaudhury's group at Lehigh, Dr. Daniel discovered that drop speeds can be tuned by adjusting the driving frequency of the vibration to match the natural resonance modes of the drop. Therefore, combining a surface energy gradient and symmetric vibration of the support affords excellent control of the drop direction, position, and its speed of migration on the surface. Furthermore, a combination of a uniformly hydrophobic surface and an asymmetric vibration also allows excellent drop control, with the added feature that drop motion is reversible, depending on the drop size and asymmetry of the driving oscillation.

             These latter studies inspired the Late Professor de Gennes to describe a new kind of Brownian motion resulting from white noise vibration in combination with dry friction (in the case of solid objects on solid supports) or contact angle hysteresis (in the case of liquid drops on surfaces).

Future Directions

             Currently, we are exploring drop motion induced by vibration on various substrates. In these studies, we are examining new ways to perform containerless chemistry experiments for biological applications that minimize fouling of the substrate.

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