01 / Qualitative handling illustration
How do free particles become mobile?
Controlled handling can put a free particle into motion and may help make it observable. When the container stops, residual liquid flow can continue and carry the particle. The wall-adherent and settled examples here remain in place.
The suitable movement depends on the product, fill level, container, and particle. A stylized swirl is shown—not an instruction to use a particular amplitude, speed, inversion count, or waiting period. No detachment or detection is guaranteed.
Interpretation limits: Free-particle transport and residual liquid flow are qualitative. Adherent or settled material need not mobilize. Use an approved, product-specific handling method; this is not an SOP.
FDA draft guidance · USP <1790> · NASA / liquid motion
02 / Idealized Stokes terminal settling
What does higher viscosity change?
For one isolated, rigid sphere settling in a still Newtonian liquid at low Reynolds number, increasing dynamic viscosity slows terminal motion relative to the liquid. The comparison holds physical diameter, density difference, and gravity fixed.
Stokes' model is vₜ = (ρₚ − ρₗ) g d² / (18 μ). A 5:1 viscosity ratio gives a modeled 1:5 speed ratio under these assumptions. This does not tell you how long a viscous formulation will circulate after swirling or when to inspect it.
Interpretation limits: Isolated rigid spheres, quiescent Newtonian liquid, low Re. Walls, interacting particles, non-Newtonian rheology, and unsteady flow are not modeled. Drawings and playback time are not to scale.
MIT / Stokes settling · Rheology / wall effects · USP <1790>
03 / Qualitative gravity–buoyancy balance
Why can a solid sink, rise, or remain suspended?
In still liquid, a fully submerged denser sphere has a net downward body force; a lighter sphere has a net upward body force. Equal particle and liquid densities give zero density-driven vertical force in this idealized view.
The animation first shows this still-liquid tendency, then shows flow carrying all three spheres. Density matching does not mean immobility in moving liquid. Trajectory does not identify particle material, origin, or regulatory category.
Interpretation limits: Illustrative spheres only. No calibrated speeds or density values; no Brownian motion, wall interactions, adhesion, or stratification. Bulk-flow transport is separately stylized.
OpenStax / buoyancy · MIT / Stokes settling · USP <1790>
04 / Idealized Stokes size comparison
Does a larger sphere settle faster?
In the low-Reynolds-number Stokes model, terminal speed is proportional to physical diameter squared. For otherwise equal isolated spheres, twice the diameter gives four times the modeled terminal speed.
The larger sphere is drawn twice as wide to explain the comparison, but both are enlarged for legibility. No actual particle dimensions or universal visible-size boundary are implied. Fibers, flakes, irregular aggregates, bubbles, and droplets are outside this comparison.
Interpretation limits: Same density difference, viscosity, gravity, and low-Re isolated-sphere assumptions. Motion begins at terminal speed. Size alone does not determine visibility; this is not a detection threshold.
MIT / Stokes settling · Visible-particle review · USP <1790>
05 / Qualitative ambiguity demonstration
Can direction prove bubble or particle identity?
No. A bubble may rise, and a solid can also rise if it is buoyant or carried by flow. This deliberately paired view shows both rising: motion is a clue, not proof of identity. Gas bubbles are excluded from the cited particulate definitions.
Bubble paths and rates depend on size, fluid properties, and interfacial conditions. Not all bubbles disappear, and not all solids sink. Hollow and filled symbols are explanatory labels, not an observation-based identification rule.
Interpretation limits: Stylized paths, not an equal-speed prediction or a classifier. The solid example is buoyant in this view. Follow the approved, product- and site-specific inspection procedure.
Bubble-motion review · OpenStax / buoyancy · FDA draft guidance
06 / Qualitative contrast comparison
Why can moving particles be harder to distinguish?
Changing the viewing field changes the apparent separation of a light or dark reference particle from its background. A visually obscuring clouded view can also soften that separation. The particles and paths remain the same in both panels.
Black and white viewing fields are recognized elements of compendial visible-particle examination conditions. Here, the clouding is only a graphic overlay—not a measured turbidity value, a formulation model, or a prediction of inspector performance.
Interpretation limits: Illustrative contrast comparison—not an inspection-performance simulation. Lower visual distinction is not proof of absence. No visibility threshold, detection probability, or accept/reject judgment is shown.
FDA draft guidance · USP Stimuli / backgrounds · Scattering / contrast review · USP <1790>