duck

        // find current surface angle
        RaycastHit hit;
        Ray ray = new Ray(transform.position,Vector3.down);

        LayerMask layerMask = ~(1 << gameObject.layer);

        if (Physics.Raycast(ray, out hit, 100, layerMask))
        {

            //Basically, get the angle between transform.up and Vector3.up.
            float angleDiff = Vector3.Angle(transform.up, hit.normal);

            // get its cross product,
            //(which is the axis of rotation to get from one vector to the other.)
            Vector3 cross = Vector3.Cross(transform.up, hit.normal);

            //Finally, apply torque along that axis according to the magnitude of the angle.
            rigidbody.AddTorque(cross * angleDiff * 10000);

        }