Evaporation rate of drop arrays within a digital microfluidic system

One essential advantage of digital microfluidic systems such as, for instance, drop-based lab-on-a-chips, is a massive parallelization of biochemical functions achieved by moving drops under surface acoustic waves or electrowetting on dielectrics (EWOD). This paper aims at characterizing the evaporation rate of a population of drops in a microsystem. Up to now, and despite its importance for end-users, the evaporation rate of one target drop selected among a population of drops has not been measured. This is essentially due to the difficulty of developing imaging in confined microfluidic drop arrays. In this paper, interferometry together with coplanar electrowetting are proposed as a new (non-imaging) evaporation rate measurement method fully compatible with digital microfluidic systems, and easy to be integrated into closed or open geometries. We investigate the impact on drop evaporation of different arrangements of drops and for different degrees of confinement. Use is made of dual-frequency electrowetting [1]: a classical actuation at a high-frequency (15 kHz) allows to set the contact angle equal to 90° while the low-frequency actuation (100–500 Hz) is responsible for the excitation of a perfectly spherical standing capillary wave along the drop surface. By considering natural frequencies of the oscillating drop and its capillary spectrum, it is possible to measure a spectral shift under evaporation. The evaporation law by Picknett and Bexon [2] is found valid even for a matrix of drops.