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Branched Flow

Short answer: Branched flow happens when waves pass through a weak, smoothly varying disorder whose features are larger than the wavelength. The waves form channels of higher intensity that keep dividing.

It was first seen with electrons and microwaves. In 2020, Patsyk, Sivan, Segev and Bandres reported the first observation with light in Nature. They sent a laser into a soap film, which acts as a thin waveguide, and the film's smooth thickness variations focused the light into branching filaments that stayed unexpectedly collimated.

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In collections:Everyday Physics

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What should you know about Branched Flow?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Branched flow of light was first observed in 2020, in soap films.

    DD-739-F01 · Write-up · Exhibit B (First in light)

  2. Branched flow was first seen with electrons.

    DD-739-F02 · Write-up · Exhibit A (The phenomenon)

Video transcript: Branched Flow: A Laser Inside a Soap Film Splits Into Branching Filaments

DID YOU KNOW? A thin soap bubble film can make branched flow visible in ordinary laser light. As the laser travels through the film, tiny thickness variations in curved regions of the soap membrane act like a constantly changing network of microscopic lenses, slightly redirecting different parts of the beam. Instead of spreading smoothly, the light concentrates into bright filaments that repeatedly split and merge, creating a tree-like pattern known as branched flow. The effect comes from wave propagation through a complex, disordered medium. NOW YOU KNOW!

Burned-in captions, read frame by frame (see transcript.md) · 29.1s Reel ·

Video coming soonBranched Flow · Instagram Reel
Branched Flow: A Laser Inside a Soap Film Splits Into Branching Filaments Instagram Reel · · 29.1s

The write-up

A laser inside a soap film splits into branching filaments. First seen for electrons; first shown for light in 2020.

  1. Exhibit A

    The phenomenon

    When waves cross a weak disordered landscape whose features are larger than the wavelength, they form channels of higher intensity that keep dividing. This is branched flow. It was first seen for electrons and in microwave cavities, and it has been suggested as a focusing mechanism for ocean waves.

  2. Exhibit B

    First in light

    In 2020, a team from the Technion and the University of Central Florida reported the first observation of branched flow of light, in a laser launched into a thin soap membrane.

  3. Exhibit C

    How the film steers light

    The film works as a slab waveguide, with the light traveling inside the membrane. Its smooth thickness variations, from about 5 nm up to several micrometers, change the effective refractive index and act as a correlated disordered potential.

  4. Exhibit D

    Surprisingly straight

    Counterintuitively, despite the random medium, the filaments stay collimated along their paths. In a curved bubble, the overall curvature adds extra focusing and makes the branches run more parallel.

  5. Exhibit E

    Seeing the landscape

    The researchers mapped the film's thickness using red, green and blue light, which reflects from the film as interference colors. They found that air movement makes the thickness landscape shift over time.

Questions people ask about Branched Flow

In which kind of wave was branched flow first observed?

It was first seen for electrons, then microwaves.

Source: Write-up · Exhibit A (The phenomenon) · Patsyk et al. (2020), Observation of branched flow of light, Nature

When was branched flow of light first reported?

In Nature in 2020.

Source: Write-up · Exhibit B (First in light) · Patsyk et al. (2020), Observation of branched flow of light, Nature; Nature Video (2020): A laser show in a soap bubble

How does the laser travel in the experiment?

The film acts as a slab waveguide.

Source: Write-up · Exhibit C (How the film steers light) · Technion (Segev group): Observation of branched flow of light (PDF); Patsyk et al. (2020), Observation of branched flow of light, Nature

What creates the disorder?

Thickness variations change the effective refractive index.

Source: Write-up · Exhibit C (How the film steers light) · Technion (Segev group): Observation of branched flow of light (PDF); Patsyk et al. (2020), Observation of branched flow of light, Nature

What surprised the researchers about the filaments?

Despite random disorder, the filaments remained collimated.

Source: Write-up · Exhibit D (Surprisingly straight) · Patsyk et al. (2020), Observation of branched flow of light, Nature

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. In which kind of wave was branched flow first observed?

    1. Light
    2. Electrons
    3. Sound
    4. Gravity waves
    Show the answer

    Answer: B. Electrons It was first seen for electrons, then microwaves. Source: Write-up · Exhibit A (The phenomenon) · Patsyk et al. (2020), Observation of branched flow of light, Nature

  2. When was branched flow of light first reported?

    1. 1905
    2. 1970
    3. 2020
    4. 2025
    Show the answer

    Answer: C. 2020 In Nature in 2020. Source: Write-up · Exhibit B (First in light) · Patsyk et al. (2020), Observation of branched flow of light, Nature; Nature Video (2020): A laser show in a soap bubble

  3. How does the laser travel in the experiment?

    1. Inside the soap film, as a waveguide
    2. Through the air beside it
    3. Reflected off the surface only
    4. Through water below
    Show the answer

    Answer: A. Inside the soap film, as a waveguide The film acts as a slab waveguide. Source: Write-up · Exhibit C (How the film steers light) · Technion (Segev group): Observation of branched flow of light (PDF); Patsyk et al. (2020), Observation of branched flow of light, Nature

  4. What creates the disorder?

    1. Smooth thickness variations in the film
    2. Dust particles
    3. Magnetic fields
    4. Temperature of the laser
    Show the answer

    Answer: A. Smooth thickness variations in the film Thickness variations change the effective refractive index. Source: Write-up · Exhibit C (How the film steers light) · Technion (Segev group): Observation of branched flow of light (PDF); Patsyk et al. (2020), Observation of branched flow of light, Nature

  5. What surprised the researchers about the filaments?

    1. They stayed collimated
    2. They changed color
    3. They stopped the laser
    4. They made sound
    Show the answer

    Answer: A. They stayed collimated Despite random disorder, the filaments remained collimated. Source: Write-up · Exhibit D (Surprisingly straight) · Patsyk et al. (2020), Observation of branched flow of light, Nature

Cite this case file

Easterling, Marcus. "Branched Flow." The Detail Department (DetailEnjoyer), case file DD-739, last updated Sep 14, 2026. https://detailenjoyer.com/case/branched-flow/

Permalink: https://detailenjoyer.com/case/branched-flow/ · Last updated

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Cite a single fact by its ID, for example DD-739-F01 → https://detailenjoyer.com/case/branched-flow/#DD-739-F01

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