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LEARNING LIBRARY

Why two streams do not mix instantly

Explore how solutes move between adjacent streams in a simple channel.

Beginner · About 3 minutes

What you’ll learn

  • Distinguish fluid transport along the channel from diffusion across an interface.
  • Understand why a junction does not guarantee complete mixing.
  • Recognise when a drawing needs a physical model.
Two miscible streams and a broadening interface A qualitative co-flow sketch. A cyan upper band and green lower band share a transition region that widens downstream. No concentration, time or mixing efficiency is calculated. FlowStream AStream BThe shared interface broadens downstream
Qualitative co-flow illustration. No concentration or mixing efficiency is calculated.

Open the related interactive explorer

When two miscible liquids flow beside each other in a microchannel, molecules diffuse across their shared interface. In a simple co-flow illustration, the transition region broadens downstream instead of showing instant complete mixing at the junction.

In a simplified model, changing the time available for diffusion changes the visible blending. Real behaviour also depends on the molecules, fluid flow and three-dimensional channel geometry. ANFF's simulation seminar identifies fluid flow, mixing and molecular diffusion as topics for computational modelling.

The straight-channel sketch in this guide illustrates a gradually broadening interface; it is not a calculated concentration field. The winding channel in the interactive Explorer shows routing only. Its colours do not depict a diffusion profile, and a curved channel should not automatically be labelled a fully effective mixer.

Limits of this overview

  • Applies to a simple co-flow example with miscible liquids; immiscible fluids behave differently.
  • No CFD, diffusion coefficient, geometry-specific mixing index or validated performance prediction is provided.
  • Animation speed is for readability and does not represent physical time.

Sources & review

  1. Transverse transport of solutes between co-flowing pressure-driven streams for microfluidic studies of diffusion/reaction processes Salmon and Ajdari; author manuscript on arXiv; J. Appl. Phys. 101, 074902 (2007) · Checked 14 Sep 2026

    Adjacent miscible streams, transverse diffusion and three-dimensional velocity effects

  2. ANFF Seminars: CFD simulation and modelling Melbourne Centre for Nanofabrication / ANFF · Checked 14 Sep 2026

    ANFF educational coverage of fluid flow, microfluidic mixing and molecular diffusion

Source checked 14 Sep 2026 · Biochips source desk (AI-assisted source check). Specialist review: not recorded. How we review information.

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