The screen material's coefficient of friction sets the physical resistance as the nib slides

What it is

When the nib slides on glass, tangential resistance comes mainly from the contact pair’s coefficient of friction (and normal force). Skin and joints read that resistance as “paper-like or not, can I brake”. It is a mechanical channel, not latency or resolution. The same pen can meet quite different resistance on polished glass, etched glass, and a paper-like film, and the feel follows.

Why it happens

Dry friction is roughly F = µN, with µ set by the nib polymer (or felt) against glass or coating, and N the writing force. Glass µ is usually low, so the nib skates; small heading corrections lack damping and strokes overshoot. Etching or frosting turns contact into many asperities, raising effective µ and adding a little stick–slip. Paper-like films use fibre or particle coats to mimic paper shear. µ is not constant: speed, temperature, sebum, and wear change it. The gap between static and kinetic friction makes a “stick then go” at stroke start, which some people read as paper bite and others as a hitch. Finger oil on the screen drops µ locally, so feel is uneven on one page. Resistance does not go through the digitizer; firmware cannot “raise friction” in software. What can be done is change materials, or fake resistance with in-pen vibration—another channel, not a substitute for real shear.

Studying it

Pull the nib across the surface on a force gauge at several normal forces and speeds, and compute µ. Have people write the same characters and make the same short-line hard stops on several surfaces.

Independent variables: surface (polished / etched / film), nib material, normal force, speed. Dependent measures: µ, start-of-stroke stick–slip, overshoot length on a stop, subjective resistance.

Report surface and nib as a pair, not “this screen’s friction” alone.

Where it stops holding

Moisture and oil can drop µ toward lubrication; even good etching then fails. A worn nib changes the pair; data from a new pen on an old film do not last. Tilted side-stroke changes contact area and effective resistance without changing µ. Gloves block shear at the skin, so resistance feel falls.

Applying it

• For writing-first devices, pick a microstructured surface or an acceptable film; do not ship only showroom-polished glass.
• Choose and replace nib and surface as a pair; when worn, replace both.
• Accept resistance with short-line stops and slow circles: overshoot on polish should clearly exceed etched/film.
• How to check: same pen, same passage, swap surface; measure stop overshoot and “I can brake”. Measure again after wiping oil, so dirt is not counted as material.

Related

• Same group: C5.13.2 A too-smooth surface lacks paper friction and lets strokes run · C5.13.3 Nib material and screen friction jointly set feel · C5.13.4 Feel is subjective and cannot be captured by latency and accuracy alone
• Adjacent: C5.01 Pressure Sensitivity · C5.12 Visual Offset Between Nib and Ink
• Search: coefficient of friction · nib resistance · etched glass