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Kaye Effect: Why Some Shampoo Streams Jump Back Up (Field Guide)

2026-03-08 · physics

Kaye Effect: Why Some Shampoo Streams Jump Back Up (Field Guide)

Date: 2026-03-08
Category: explore
Domain: physics / fluid dynamics / rheology

The 10-second picture

Pour some shampoos or liquid soaps in a thin stream onto a puddle of the same fluid, and sometimes the stream does something bizarre:

That jump is the Kaye effect.

Why this is interesting

This is one of those tabletop phenomena that looks like a party trick but forces you to think about three real physics layers at once:

  1. Non-Newtonian rheology (shear-thinning + viscoelasticity),
  2. Interfacial lubrication (the role of a very thin air layer),
  3. Fast geometry changes (heap, dimple, loop, collapse) on millisecond timescales.

It is a compact example of how "macroscopic weirdness" can come from microscopic flow structure.

Short history (and why the interpretation evolved)

1) First report (1963)

Alan Kaye reported the effect in Nature as "A Bouncing Liquid Stream" while pouring polyisobutylene solution in decalin.

2) Revisit with faster filming (1976)

Collyer & Fisher revisited the phenomenon with higher-speed filming and described the emergent stream as a loop-like structure during the event.

3) Modern high-speed analysis (2006)

Versluis et al. showed the effect in common fluids (including shampoos/soaps), framed it as a continuous-flow phenomenon, and demonstrated stable/directed variants.

4) Mechanistic update (2019)

King & Lind reported evidence that the effect disappears in vacuum, supporting that an air layer is essential for slip/lubrication. They also argued viscoelasticity plays a key role in enabling/sustaining the jump.

Takeaway: the field moved from a "just shear-thinning" story toward a coupled air-lubrication + rheology story.

Working mechanism (practical mental model)

Think of one jump event as a short life-cycle:

  1. Impact + heap formation
    The incoming jet builds a small viscous heap at the impact region.

  2. Dimple / slip zone appears
    A local geometry forms where the incoming stream can slide rather than instantly merge.

  3. Outgoing streamer is ejected
    Part of the flow is redirected sideways/upward as a leaping ribbon/stream.

  4. Self-limiting collapse
    Geometry changes (and flow partitioning) shut off the condition that sustained the jump, so it collapses.

What controls success is not a single parameter, but whether the system can maintain a low-dissipation sliding interface long enough for a coherent outgoing jet to form.

What seems to matter most

1) Fluid rheology window

You generally need fluids in the right non-Newtonian regime:

This is why everyday shampoos/soaps can show it, but many simple Newtonian liquids do not.

2) Air availability at impact

The vacuum result is a strong clue: if air cannot be entrained/maintained at the interface, the jump is suppressed.

3) Geometry + forcing window

Height, flow rate, nozzle/stream thickness, and substrate condition create a narrow operating region. Outside that region you get only:

Common misconceptions

If you want to reproduce it at home (safe version)

Use:

Tips:

Safety:

Why this matters beyond a cool demo

Kaye-effect physics maps to real engineering themes:

The big lesson: in complex fluids, interface physics can dominate bulk intuition.

One-line takeaway

The Kaye effect is a short-lived, self-organized jumping-flow state created by the right marriage of non-Newtonian rheology and air-layer lubrication at impact.

References