When the solid has a high affinity for water - in which case it is called hydrophilic (high energy e.g. glass)- water spreads. In the opposite case of hydrophobic (low energy e.g. teflon) surfaces, water does not spread but, instead, forms at equilibrium a spherical cap resting on the substrate with a 'contact angle'.
Infections such as trichomoniasis, gonorrhea, and Chlamydia (all STDs); and yeast infection can all cause vaginal discharge. Vaginal discharge can be associated with other symptoms such as burning or itching. Antibiotics are given for infections that cause vaginal discharge.
It's normal to have a clear and/or white discharge. The vagina is self-cleaning. Since it's coming out of you yellow, you need to see a doctor as you probably have an infection. Different Types of Discharge: White: Thick, white discharge is common at the beginning and end of your cycle.