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October 2011 Moms
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? for the science nerds

StewartandmauraStewartandmaura member
in October 2011 Moms

There is a strange occurrence happening in all my "spill-proof"  straw sippies.

 While they will not spill if E shakes them or turns them upside down, if she sets it down on a table for any period of time, capillary action is drawing the liquid up the straw. Eventually, it will spill over onto the table.  If I open the lid, the liquid will return into the cup, like as if there was a pressure build-up inside the cup forcing the liquid out.

Is it because there is no escape for the expanding air as the contents heat up to room temp? I am thinking I need to find a way to vent it.

Does this happen to anyone else?

I know I am overthinking the science behind this, but it is a PITA. And it totally defeats the purpose of having spill-proof cups.



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Re: ? for the science nerds

  • mamaZbomamaZbo member
    Ours does the same thing. I think you are right with your reasoning too. Maybe open the lid after she initially drinks out of it and see if you can equalize the pressure a little bit. We have the flip top straw kind and I try and close it if it sits for more than a few minutes which also helps. I can't imagine how to vent it without causing it to spill when she shakes it.
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  • DoctorWorm222DoctorWorm222 member

    I'm no physicist, but ...

    Is it possible that your LO is blowing into the straw? I've watched E with her straw cup, and it seems like she sucks then sort of blows back into it. With every sip I can see a little bit of the fluid going up the straw and then some (carrying the debris from her mouth) goes back into the cup. I think the pressure in the cup is what's forcing the liquid up through the straw. 

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  • missyisheremissyishere member
    imagecrystalbaby:

    Temperature difference is what is causing the liquid to be drawn up the straw. I only use water in the straw cups because of that issue. V didn't like the other sippy cups at all, but I forced them on her because I was tired of cleaning up milk everywhere.

    ETA: Temperature difference between the liquid in the cup and the ambient air. 

    I've been struggling with this for several months now.  Nora will not drink out of a regular sippy.  I tried several, and they never seemed to take.  I was also sick of buying different sippies to test it out.  She has loved the straw sippies for a while now.  I just try to ensure she drinks them quickly before the milk has a chance to warm.  Also, try not to tighten the lid too much.  The pressure seems to build faster that way. 



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  • ScramblersmomScramblersmom member
    I agree with PP on temperature and pressure. I would probably just leave the lid open at all times.
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  • EloiseWeenieSkipperdeeEloiseWeenieSkipperdee member
    I've been using camelbak kids cups, and it has helped a lot (meaning- helped me with not wiping up milk all the time). It will still suck up milk in the straw, but the mouthpiece shuts it off.

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