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Workflow-Induced Shear as a Hidden Source of Variability in Particulate Systems: Introduction of Substantially Zero-Shear Protocols
Summary
Turns out the common lab habit of shaking, stirring, or vigorously mixing liquids can actually mess up how tiny particles—like microplastics—behave, causing them to clump together in ways that throw off test results. Researchers found that skipping the shaking (letting liquids sit still instead) gives more accurate, consistent readings, which matters for anything from detecting microplastics in food and water to preparing baby formula and medications correctly. This suggests that simply changing how we mix things during manufacturing or testing—not just what we mix—could improve the reliability of products and tests that affect our health.
Mechanical agitation is conventionally viewed as essential for uniform mixing and reproducibility in laboratory, pharmaceutical, nutritional, industrial, and consumer workflows. Systematic experiments conducted during development of a surface-area-based optical microplastics/nanoplastics detection assay revealed the opposite: shear-inducing steps (stirring, vortexing, inversion, pipetting) dramatically increase forced particle collision frequency, leading to nonlinear aggregation, collapsed concentration resolution, and loss of reproducible optical endpoints — even when analyte concentrations are precisely known. These observations led to the development of substantially zero-shear preparation protocols, in which reagents are introduced into a stationary liquid with no subsequent mechanical agitation (shear rate < 0.5 s⁻¹). Under zero-shear conditions, natural diffusion- and surface-area-mediated interactions predominate, preserving concentration-dependent behavior and producing stable, characteristic optical endpoints (including ring-formation patterns). Quantitative reproducibility metrics — the Shear Variability Index (SVI), Dissolution Variability Index (DVI), and Combined Variability Estimate (CVE) — were introduced to model workflow-induced variability. The zero-shear framework identifies workflow mechanics themselves as a previously unrecognized dominant variable in interaction-driven particulate and colloidal systems. It offers a simple, broadly applicable strategy for improving analytical consistency, simplifying protocols, and enhancing reliability across microplastics/nanoplastics detection, pharmaceutical reconstitution, infant nutrition, agricultural tank mixes, biologics formulation, surfactant systems, and automated laboratory workflows. (Public disclosure document: Nonprovisional 19/672,559)