Heterogeneous nucleation is very important for the formation of dropwise condensation,and it is directly related to the surface properties of materials.It is therefore necessary to study the relation between material surface topography and liquid nucleation quantitatively.The most difficult thing is,however,how to record the initial nucleation sites of liquid drops.Magnesium was used as the condensation surface in this study since it can react with hot water (condensate),generating magnesium hydroxide.The variation of the chemical compositions on the surface can be used as the marker of initially-formed condensate drops because this variation only takes place at the places where the initial condensate nuclei and liq uid drops form.