# Branched Flow (DD-739) > 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. - Department: Science - First published: 2026-09-14 - Last updated: 2026-09-14 - Permalink: https://detailenjoyer.com/case/branched-flow/ - Author: Marcus Easterling (DetailEnjoyer), The Detail Department ## On the timeline - **2020**: Patsyk, Sivan, Segev and Bandres report the first observation of branched flow with light, in Nature Full timeline: https://detailenjoyer.com/timeline/?case=branched-flow ## Quick facts - **DD-739-F01**: Branched flow of light was first observed in 2020, in soap films. _(Write-up · Exhibit B (First in light))_ - **DD-739-F02**: Branched flow was first seen with electrons. _(Write-up · Exhibit A (The phenomenon))_ ## Write-up A laser inside a soap film splits into branching filaments. First seen for electrons; first shown for light in 2020. 1. **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. **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. **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. **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. **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. ## Related case files - [Marangoni Motion (DD-024)](https://detailenjoyer.com/case/marangoni-motion/): More physics in thin liquid films: surface tension pulling a film of wine up the glass. - [Cherenkov Glow (DD-020)](https://detailenjoyer.com/case/cherenkov-glow/): Another striking optical effect: light from charged particles outrunning light in water. - [Central Limits (DD-588)](https://detailenjoyer.com/case/central-limits/): I linked another case where order emerges from randomness - [Around the Rainbow (DD-651)](https://detailenjoyer.com/case/around-the-rainbow/): More patterns made by light ## Reel vs. record - **Mechanism**: the laser is launched into and travels inside the film, which acts as a waveguide. It isn't shone through the bubble from outside. - **Cause**: the branching comes from the film's smooth thickness variations, which act as a disordered refractive-index landscape. Curvature only adds extra focusing that makes the branches more parallel. - **Missing context**: branched flow was first observed for electrons and microwaves. Seeing it in light was a first, reported in Nature in 2020 (Patsyk, Sivan, Segev, Bandres). - **Missing context**: counterintuitively, the filaments stay collimated (nearly straight) despite the random medium. - **Framing**: 'microscopic lenses' is a helpful analogy. The researchers describe the effect as a correlated disordered potential whose features are larger than the wavelength. ## Video: Branched Flow: A Laser Inside a Soap Film Splits Into Branching Filaments - Duration: 29.1 s · Published: 2026-09-14 - File: https://detailenjoyer.com/assets/videos/DdQRqlWRNOg.mp4 ### Transcript 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! ## Questions & answers **Q: In which kind of wave was branched flow first observed?** A: 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)_ **Q: When was branched flow of light first reported?** A: 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)_ **Q: How does the laser travel in the experiment?** A: 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)_ **Q: What creates the disorder?** A: 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)_ **Q: What surprised the researchers about the filaments?** A: 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)_ ## Sources - [Patsyk et al. (2020), Observation of branched flow of light, Nature](https://www.nature.com/articles/s41586-020-2376-8) - [Nature Video (2020): A laser show in a soap bubble](https://www.nature.com/articles/d41586-020-01991-5) - [Technion (Segev group): Observation of branched flow of light (PDF)](https://phsites.technion.ac.il/publications/msegev/Nature-583-60-2020.pdf) ## 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/