41. The contact angle the droplet had immediately before advancing outward is termed the advancing contact angle . 42. The contact angles at which spreading / imbibition occurs are between 0 and ? / 2. 43. Human skin, for example, is a hydrophobic surface with a contact angle of about 90 degrees. 44. Where \ theta is the contact angle . 45. Choice of \ scriptstyle \ hat n depends on which fluid the contact angle is specified for. 46. The contact angle the droplet surface makes with the solid horizontal surface decreases from left to right. 47. Surface energy can be estimated by contact angle measurements using a version of the Young Laplace equation: 48. The contact angles obtained for hydrophobic surfaces are directly related to surface roughnesses by the Wenzel equation. 49. Bird feathers and lotus leaves are considered superhydrophobic, with contact angles of 150 and 170 degrees, respectively. 50. The thickness of a puddle of liquid on a surface whose contact angle is 180?is given by:
