model with different friction

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I haven't tried this, but I would look at creating multiple rectangular models, each having different friction properties, then position instances of these models so they form a continuous surface. If you make the z-dimension very short, you could even place them on top of a standard ground plane (so there's no edge-of-world for your robot to fall off of.)

Follow-up thought: I assume you are aware that gazebo's ODE friction calculations are very crude. Don't be surprised if your simulation results seem quite unrealistic.