Viscosity, and how it reaches the customer
Updated
Viscosity is treated as a sensory property and it is also the thing that decides whether your product can leave the container you bought.
Viscosity is a packaging specification
A pump has a viscosity range it can move. A dropper needs the product to flow through a narrow orifice under gravity. A tube needs it thick enough not to run and thin enough to squeeze. An airless system needs it to follow the plate without cavitating.
Which means the container is not chosen after the formula is finished. If a serum thickens slightly between development and production, the dropper stops working and the whole run is unsellable in the packaging you already own.
What sets it
The emulsifier system and the thickener, mostly, plus the oil phase percentage and the internal phase volume. Gums and polymers do the heavy lifting, and each brings its own feel: slip, tack, cushion, and how it breaks under a finger.
Temperature matters enormously during manufacture and rather less on shelf, which is why a batch that looks wrong while cooling can be right the next morning.
Why it moves
It builds after manufacture, sometimes over days, as the structure sets. It changes with temperature, which means a product measured warm reads differently from the same product at 25 °C. And it drifts over shelf life, which is one of the attributes a stability study tracks precisely because it is an early indicator that something is moving.
Electrolytes affect it too, so the same salts that lower effective HLB will also thin an emulsion.
Specifying it
Measure at a defined temperature, a defined time after manufacture, and record a range rather than a number. Then check the range against the packaging supplier's stated viscosity window before ordering.
And re-measure after any reformulation, because a change made for preservation or for pH frequently moves viscosity as a side effect, and the dropper will be the thing that tells you.