Hydrophilicity is a critical feature in flow dynamics, especially in a passive flow device. Hydrophilic surfaces are water-loving, they allow aqueous fluids to flow and wet out across the surface, enabling them to move through the channels. The surface energy or degree of hydrophilicity varies widely in materials from water-repelling or hydrophobic (low surface energy), where the water dropped on the surface forms an almost perfectly round bead to, water-loving or hydrophilic (high surface energy) where water wets out across the surface. The amount of hydrophilicity needed will be dependent on the requirements of your device, including the properties of the fluid medium, channel design, capillary effect, enzyme or reagent chemistry and reaction time needed. The hydrophilicity of a given surface, can be measured using a goniometer. This instrument measures the angle that a drop of water creates on a surface. The smaller the angle the drop of water creates with the surface the more hydrophilic the surface. Generally, a contact angle smaller than 90 degrees is considered hydrophilic and if the contact angle is larger than 90 degrees it is considered hydrophobic².