I got a latte with whipped cream this morning, and towards the end of the drink I noticed a peculiar effect.

Regardless of how I turned the cup and thereby the coffee, the whipped cream pieces didn't move in position, only floated or stuck to the bottom depending on whether or not there was coffee in the bottom. You can take a look at a video I took to see this effect directly.

enter image description here

What is going on here? What causes it to stay in the same position as opposed to moving with the fluid flow?


closed as primarily opinion-based by Jim, ACuriousMind, Danu, Kyle Kanos, John Rennie Mar 31 '15 at 17:22

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ they look like they are stuck to the cup. $\endgroup$ – Phoenix87 Mar 31 '15 at 14:09
  • $\begingroup$ @Phoenix87 They are when the coffee is no longer below them, but are afloat when the coffee goes beneath it $\endgroup$ – Zach Saucier Mar 31 '15 at 14:10
  • $\begingroup$ There are too many possible explanations for this effect. As such, I'm voting to close this as primarily opinion-based because answers would inevitably provide their opinion of the specific mechanism and not be based on experimentally verified fact. All guesswork in determining the cause $\endgroup$ – Jim Mar 31 '15 at 14:21
  • $\begingroup$ @Jimnosperm Fair enough, I don't know enough about it to say if you're wrong. Is there anything I could do to make it more specific? $\endgroup$ – Zach Saucier Mar 31 '15 at 14:24
  • 2
    $\begingroup$ +1 Because of the video. It demands a (simple) explanation. (Like how the fluid actually moves.) (Also, the pictures aren't very helpful, I think.) $\endgroup$ – Keep these mind Mar 31 '15 at 16:54

If the coffee is cold enough then it will make a layer of whipped cream, which is basically a mixture of sugar and coffee. The thing is that the layer becomes sticky at the surface while the coffee is liquid beneath and flows in the lower layer. The liquid flows because the lower layer is not dry enough and sticks at the surface to resist the liquid layer while the liquid is being shaken.

  • $\begingroup$ I'm sorry, I don't understand everything you're suggesting here. Could you try to rephrase what you're saying? $\endgroup$ – Zach Saucier Mar 31 '15 at 16:41

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