A method of measuring the spreading, wetting, slipping, sticking, edge effects, friction, and lubricity of a test substance comprising;
- providing an apparatus comprising a movable transport plate assembly securely mounted to, and constrained to move in only one dimension on, a polished linear rod;
- a movable lower substrate having a lower test surface, the lower substrate securely mounted on top of the transport plate assembly;
- a lower substrate holder mounted on said movable transport plate assembly and configured to removably and securely attach said lower substrate to said movable transport plate assembly;
- a stationary upper test assembly having an upper side which is perpendicular to the upper test surface;
- a stationary upper substrate having an upper test surface, the upper substrate securely mounted to the upper test assembly;
- an upper substrate holder mounted to said upper test assembly and configured to removably and securely attach said upper substrate to said upper test assembly;
- a force conducting rod having a first end directly connected to the side of the upper test assembly and extending parallel to the upper test surface;
- a force transducer directly connected to a second end of said force conducting rod, said force transducer being configured to measure a force applied to the force conducting rod by the upper substrate;
- a vertically elongated mount which secures the force transducer, force conducting rod, and upper test assembly to the base;
- a first normal force applied to the upper test surface, by which a second normal force, equivalent to the first normal force, is created between the upper and lower substrates;
- a torque regulated motor configured to drive the movable transport plate assembly and lower substrate at a speed relative to the stationary upper substrate, a distance between said motor and said base being adjustable;
- a position encoder configured to record a position of the movable transport plate assembly and lower substrate;
- a position encoder configured to record a position of the movable transport plate assembly and lower substrate;
- a controller configured to control the torque regulated motor such that the speed and the position are maintained within 0.1% of a target speed and target position, respectively;
- and a data acquisition interface which receives and records each of the force, the torque, the speed, and the position as a function of time;
- providing said test substance provided between and in direct contact with each of the upper and lower substrates;
- varying the speed at a frequency such that the position oscillates about a center position;
- recording the force and torque as a function of time throughout the step of varying;
- and
- analyzing the recorded force, speed and torque as a function of time to determine the spreading, wetting, slip-stick, edge effects, friction, and lubricity of the test substance.