You have to go way beyond "material".

There is ZERO Normal Force when you have something trying to climb a window.  Look up friction and Normal Force in your physics book if you don't know what I just said.

I suggest you google "window climbing robots".  There are a truckload of them some quite cheap too.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

Its ability to generate normal force is not the problem. That is being done by using the venturi effect. Its all about it moving around the window.

Okay. Try this:
http://www.dowcorning.com/content/rubber/rubberprop/mech_friction.asp

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

You might try non-slip drawer liner. Something like that is usually available at most hardware stores. It can be cut into appropriate sized strips and glued around your wheels.

See: http://www.google.com/products?q=non+slip+liner&btnG=Search+Products&show=dd

For best results, test several materials and pick the best. Use a fixed weight object on a piece of glass or mirror with the test material between the two and tilt the surface until you see slippage. The greater the tilt angle before slippage, the higher the coefficient of friction.

Good luck.

Rob Steves - Zaber Technologies Inc - http://www.zaber.com

using wheels to do this job.. cant you replace this wheels by a grips which attached with tools or devices that makes suction effect ( may by using any light weight and small pumps that produce a Vacuum) which is specific for your application

sorry.........i mean compressor, not pump