Tribological Systems Design - Lecture 17 - Adhesion, van der Waals Forces, Hamaker Constant

This video explains the causes of adhesion between solids. What are van der Waals forces and how can we measure it. What is Hamaker constant? How do geckos walk on a surface up side down?
[I acknowledge figures or tables, if any, taken from these books:
Engineering Tribology by John Williams
Applied Tribology by M. M. Khonsari and E. R. Booser
Principles and Applications of Tribology by Bharat Bhushan
Tribology on the small scale by C M Mates]