If I pour water in a glass to make a cup of tea, I noticed that if the water that comes out of the kettle is very hot, almost no water is spilled. If the water is cold though, much more water is spilled. The water streams over the surface of the kettle. Why does the higher velocity of the water molecules cause them to stick less to the kettle?

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    $\begingroup$ Before jumping to "why does the higher velocity of the water molecules cause them to stick less to the kettle?" I'd suggest that what's needed is a controlled experiment demonstrating that the effect is real, rather than just anecdotal (perhaps it's a consequence of you being subconsciously more careful with hot water because of the burn risk?). $\endgroup$ – Kyle Oman May 19 '16 at 11:39
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    $\begingroup$ The viscosity at 100 C is approximately 4 times smaller than at 20 C. Smaller viscosity is lower resistance in a flow. $\endgroup$ – nluigi May 19 '16 at 11:41
  • $\begingroup$ @nluigi Huh, that suggests that my hunch that this is just anecdotal may be completely wrong. Maybe expand it into an answer? $\endgroup$ – Kyle Oman May 19 '16 at 11:45
  • $\begingroup$ @KyleOman - i still agree with your comment that the experiment should be controlled. I wonder how pronounced the effect actually is then. If i should believe valerio's answer, it's an actual measurable effect. $\endgroup$ – nluigi May 19 '16 at 11:49
  • $\begingroup$ I did the experiment. With the kettle. The less water you pour, the more visible the effect. $\endgroup$ – descheleschilder May 19 '16 at 12:09

We're talking about the Coanda Effect, right? Then I think this article could provide some useful insights.

Quoting from the article:

When the fluid flows over the heated curved surface in proximity of the curved surface as the temperature of the curved surface increases, dynamic viscosity of the fluid at the vicinity of the wall is increasing with respect to the fluid which is far from the curved surface. Then the thermal heat capacity of the fluid near to surface is increasing, and then consequently, the Prandtl number of the fluid near the curved surface is increasing. In this way the momentum boundary layer would be increasing resulted in the decreased adhesion angle.

The second mechanism can also be given by assuming the constant Prandtl number. Increasing the jet velocity, the thermal gradient near the surface is increasing and the thermal boundary layer would be decreased and consequently the momentum boundary layer would be decreased. In this way the adhesion angle would be increased. Consequently, the observed earlier detachment happens by the effect of the complex equilibrium of the above-mentioned effects.

The conclusion is that

Thermal effect on the flow has been analyzed as shown in table 1. Increment of the temperature of curved surface induced the earlier detachment of the jet.

enter image description here

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    $\begingroup$ Strange. It´s so obvious now! The hot water that leaves the kettle consists of faster-moving water molecules wich causes the water to bend less around the outlet because the faster the molecules move the lesser time they have to exert a force on the kettle. I think the same will happen with a water droplet that you gently push out of a syringe. A hot droplet will fall sooner than a cold one. $\endgroup$ – descheleschilder May 20 '16 at 11:03

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