D2 Viscometry

Topic

The viscosity of this glass as a function of temperature—the subject of this model—is measured directly using two complementary techniques: the rotational viscometer and the parallel-plate viscometer. Because the glass's viscosity drops by several orders of magnitude between its melting and annealing ranges, neither technique alone can cover the entire viscosity spectrum.

The rotational viscometer measures the glass's viscosity in the high-temperature melting range: a rotor submerged in the molten glass spins at a set speed, and the torque required to maintain that rotation reveals the viscosity of the molten glass—which remains fluid enough for the rotor to turn within it—at that temperature. Higher molten glass temperatures result in lower viscosity and, consequently, lower torque requirements to spin the rotor.

In contrast, the parallel-plate viscometer measures the glass's viscosity in the annealing range, at temperatures far lower than those of the melting range: a glass sample, compressed between two parallel plates under a constant load, slowly flattens as the glass flows; the rate of flattening reveals the glass's viscosity in this range—where viscosity is too high to be measured using a rotor. Higher viscosity in the annealing range causes the sample to flatten more slowly under the same load, while lower viscosity causes it to flatten more quickly. For this reason, the combined data from the two initial techniques—covering both the melting and annealing ranges—allow for the reliable fitting of parameters for another model in the set: the one describing the glass's viscosity across its entire temperature range using the VFT (Vogel-Fulcher-Tammann) equation. Because the data cover both temperature extremes rather than just one, the parameters are reliably determined across the full temperature range described by the equation, rather than just a portion of it.

ID:4535