Boundary Techniques – Introduction

The most common flow vis technique is to mark or ‘seed’ some of the fluid, while the rest remains transparent. The seed interacts with light while the transparent fluid does not- at least not as much (more on index of refraction techniques later). The boundary between the seeded and unseeded flow is then visible, so let’s call this a ‘boundary technique’. A slight variation is to mark two adjacent fluids differently, such as pouring two paints side by side.

Most of the transparent fluids are gases or gas mixtures like air, or liquids like water. Both air and water are made of relatively small molecules; that’s why they are transparent; they don’t interact with photons in the visible light spectrum very strongly.

The substance used for seeding can be categorized by size; let’s say molecule scale (< 1 nanometer) like dye molecules versus collections of molecules large enough to be called particles if solid, or aerosols if liquid, > 0.1 micrometer or 100 nanometers. The way each of these two categories works with fluids and with light is very different. Let’s start with molecules for seed or marker; then we’ll cover particles as seeding in air and water.

Dyed mineral oil injected into clear mineral oil forms layers of vortex rings. Jacob Varhus, Get Wet, Spring 2013
Figure 1: Dyed mineral oil injected into clear mineral oil forms layers of vortex rings. Jacob Varhus, Get Wet, Spring 2013

In liquids, specifically water, we would inject a ‘dye’ (NOT die). Dyes are usually large molecules with special chromophore groups stuck on . OK, that’s all the color chemistry we’ll do. Dyes are generally used in water (think food coloring) and are discussed below. Besides water, you might want to use an oil, maybe a vegetable oil as your transparent fluid. Recently, oil-based food dyes have become available, used in chocolate and candy, and they also work in vegetable oils but you’ll need to use a lot because they aren’t very concentrated. Beta carotene, derived from carrots, is supposed to be fat soluble, but the powdered form I tried was water soluble instead. There is also a fluorescent dye you can add to motor oil or refrigerants to help find leaks , as shown in Figure 1. There are  oil or solvent based dyes used in histology and for dying leather , but these are toxic and/or highly flammable; not recommended for casual use.

You might be thinking ‘what about oil paints’? It turns out that oil paints have color because of pigments: finely ground solid particles mixed into linseed or other oils. Let’s delay discussion of paints to the particulate section.

I can’t think of any big molecules that could be used like a dye in air or other gases. Gases are just too low in density. Even a pure gas of dye molecules won’t interact with light enough to be visible in normal light; you’d need a focused laser beam. Instead particles and aerosols are used for boundary techniques in gases.


So here’s how this section on boundary techniques is organized

  • Dye (Molecular) Techniques
  • Particle Techniques
    • Particle physics: flow and light
    • Particles for seeding air
    • Particles for seeding water


Fiebings, “Fiebings Professional Oil Dye 50 2030 SDS.” [Online]. Available: [Accessed: Dec. 29, 2021]
International Agency for Research on Cancer, GENERAL INTRODUCTION TO THE CHEMISTRY OF DYES. International Agency for Research on Cancer, 2010 [Online]. Available: [Accessed: Dec. 28, 2021]
“Dye / UV - Fluorescent Dye 8 oz NTE 788101 | Buy Online - NAPA Auto Parts.” [Online]. Available: [Accessed: Dec. 17, 2021]
“Beta Carotene - 20% Beta Carotene Powder Extract - Converts Into Vitamin A,” Prescribed For Life. [Online]. Available: [Accessed: Dec. 01, 2021]
“Eating with Your Eyes: The Chemistry of Food Colorings,” American Chemical Society. [Online]. Available: [Accessed: Nov. 17, 2021]
Clouds 2: Why Are There Clouds?
Dye Techniques 1 – Do Not Disturb