Effects of wax boundaries in combination with evaporation on dynamics of fluid flow in paper-based devices

Since their introduction, paper-based microfluidic analytical devices (mu PADs) have been ubiquitously utilized for different applications. The spontaneous imbibition of liquids in the paper-based devices that eliminates the requirement of an external pumping system has played a primary role in making paper an appropriate alternative for many other materials in the fabrication of microfluidic devices. Wax patterning is one of the most common methods to fabricate mu PADs. Dynamics of the flow in channels with wax boundaries deviate from Washburn's law. Despite some research performed to model the effects of wax boundaries, some gaps remain in the presented models. A more general model is needed to account for the effects of wax boundaries and evaporation simultaneously. In this work, we present a general model to model the wax boundaries by introducing a coefficient in the equation of wicking height. We verify our model experimentally for relative humidities of 100% and 20%. Results show that our model can predict wax boundaries effect on imbibition height with considerable accuracy.