Transwell has two surfaces

A Transwell is a laboratory device used in cell culture and cell migration studies. 

It consists of two compartments separated by a semi-permeable membrane. 

These two compartments are often referred to as the apical and basolateral chambers. 

The apical chamber is the upper chamber, and the basolateral chamber is the lower chamber. 

Cells or substances can be added to one chamber and their movement or interaction with the other chamber can be studied through the semi-permeable membrane.

The specific characteristics of the surfaces may depend on the type of Transwell used and the experimental setup, but these are the two surfaces typically referred to in the context of Transwell assays.