A method for determining wettability of a solid, such as a reservoir rock material, is described. The method includes disaggregating the material, for example by grinding and placing the disaggregated material on the surface of the fluid. The wettability is analyzed based on whether a portion of the material floats on or sinks into the fluid. The method is well suited for heterogeneous solid materials that have mixed wetting characteristics and/or have varying surface types. The fluid can be evaluated as a potential treatment fluid or a component thereof that can be used for treating the rock formation. For example, the potential treatment fluid can include a surfactant or an oxidizing agent. A simple observation can be made whether substantially all of the material placed on the surface of the fluid sinks into the fluid, or the portions of floating and sinking material can be weighed.