(715d) Equilibrium Contact Angle Profiles Along Capillary Tubes: Measurements and Implications for the Analysis of Capillary Filling Dynamics

Self-driven capillary flow is a fundamental physical phenomenon exploited in many microfluidic systems to avoid the use of external pumps and enhance portability. For a liquid that wets the inner surface of a microchannel, factors such as the equilibrium contact angle (ECA) between the liquid and the walls of the channel, liquid properties (surface tension, viscosity, and density), and the geometry of the system (e.g., the diameter of a circular capillary tube) determine the volumetric flow rate obtained as a function of time. Although the Lucas-Washburn equation is commonly used to describe such capillary filling dynamics, experimental deviations from this model have been described in the literature. For instance, the contact angle is known to change during capillary filling, which has led to the formulation of several dynamic contact angle (DCA) models that are a function of the ECA among other parameters. However, both microfluidic applications and fundamental capillary rise studies employ a single ECA value to characterize an entire microchannel. Here, we use circular glass capillary tubes (200-800 microns in diameter) to illustrate how a more accurate description of the surface properties of the microchannel is an equilibrium contact angle profile (ECAP) along the length L of the capillary tube, instead of a single ECA value. Thus, we present an experimental methodology to characterize glass capillary tubes using non-invasive and non-destructive capillary rise experiments that enable ECA measurements along the capillary tube, so that an ECAP (ECA versus L) can be retrieved. In addition, the measurement of an ECAP from capillary filling experiments under flow conditions where DCA effects can be neglected is investigated. We discuss the implications of these findings in the implementation of a point-of-need rheological analysis platform, whereby capillary filling dynamics inside glass capillary tubes are recorded with a smartphone camera and probed to retrieve the rheology of the fluid.