What equation is used to evaluate normal physical contact area

Hello

When two particles are in contact with each other and there is a bit of overlap, what equation is used to determine the value of the normal physical contact area (contact.getNormalPhysicalOverlap())

is this equal to Ac in eq33 as shown in the image?

If not, how is this variable contact.getNormalPhysicalOverlap() determined? Please give me the equation or a geometrical explanation for the same