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A few months ago, I tried to tackle the problem of finding the shape, height and other properties of a puddle (whenever a few mL of some liquid are dropped in a "uniform way" so that the puddle remains approximately circular). Trying to find the height, dimensional analysis with density, gravity and superficial tension gives the "famous" (appears in most of the books about this problem) formula:

$$ H = \sqrt{\frac{\sigma}{\rho g}} $$

But as soon as I found this, I discarded it (at this point I hadn't researched anything about the problem) because it seemed to contradict the experiments I'd done so far! I had dropped a few mL of water and honey, and, as I expected, the honey puddle was higher than the water one (3 mm vs 1.5 mm approx). But this is contradictory! The surface tension of honey is similar to that of water, but the density is significantly higher, so the height should be lower.

What does this mean? Is it because the viscosity of the honey makes it "stop" from spilling on the ground and makes it stick on top of the honey that's already on the ground? I've tried a different approach which leads me to a formula that agrees much more with the experiments, but it's completely different from this one and doesn't have the theoretical justification that this one does.

Any suggestions or ideas? Thanks in advance.


EDIT: Here's what I obtained so far and some of the experiments I did (keep in mind that I didn't have much precision due to the lack of any sophisticated instruments). (All measurements were taken at about 20-25 degrees Celsius)

$$ H = \kappa \sqrt[\leftroot{-1}\uproot{2}\scriptstyle 15]{\frac{\eta^2 \sigma^6}{g^7 \rho^8}} $$ Where $\eta$ is the viscosity, $\sigma$ the superficial tension, $g$ is gravity and $\rho$ is density. The value of the constant is $\kappa = 0.89 \pm 0.24 $. With this, I can "predict" the height of a puddle of water, honey, glycerin and alcohol. $$ H_{water}^{theory} = (1.1 \pm 0.4) mm \leftrightarrow H_{water}^{experiment} = (1.5 \pm 0.5) mm$$ $$ H_{alcohol}^{theory} = (0.8 \pm 0.3) mm \leftrightarrow H_{alcohol}^{experiment} = (0.8 \pm 0.5) mm$$ $$ H_{honey}^{theory} = (3.0 \pm 1.1) mm \leftrightarrow H_{honey}^{experiment} = (3.0 \pm 0.5) mm $$ $$ H_{glycerin}^{theory} = (2.4 \pm 0.9) mm \leftrightarrow H_{glycerin}^{experiment} = (2.6 \pm 0.2) mm$$

Also, the formula I've obtained for the shape of the drop is (in two dimensions): $$ H(r) = H_m(1-e^{\frac{tan{\theta_C}}{H_m}(\left|r \right| - R)}) $$ With $R$ the radius of the puddle, $\theta_C$ the contact angle and $H_m$ the maximum height of the puddle (can be obtained by the formula before) and the radius is approximately $R = \sqrt{\frac{V}{\pi H_m}} $. An example with R=10, the angle 64 degrees and the height 2.8.

I have a formula for the contact angle but requires extreme precision, so it's not of much use itself.

$$\theta_C = \arctan \left(\frac{\pi R H_m^2}{\pi R^2 H_m - V}\left(1+ \sqrt{\frac{2V}{\pi R^2 H_m} -1} \right) \right) $$

hft
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Rafa
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    Your honey probably dries out and gets a skin before it spreads out. Did you measure the surface tension of your materials or are you just using textbook values? I don't think that's much of a precision experiment, either. – CuriousOne Jul 24 '16 at 23:30
  • are you sure theres not some disclaimer about newtonian fluids there? Honey is hardly an ideal liquid – anon01 Jul 24 '16 at 23:32
  • I don't really have a lab, I'm just a student in his house, i get the surface tension and the viscosity from internet tables but of course i keep the interval of values that my liquids can have. This is more of an order of magnitud estimate. And about the newtonian fluids thing, as far as I can remember they don't really specify it, and use it quite generally. I've seen it used in actual Phd papers for different approaches with simulations and non-uniform surfaces – Rafa Jul 25 '16 at 00:03
  • Viscosity of honey varies quite a bit with temperature - might be a factor some how. – M. Enns Jul 25 '16 at 00:27
  • The shape itself will depend on liquid-air, liquid-solid, and solid-air interaction forces, giving a radius of curvature according to laplace's formula. you can take the central radius of curvature as the height of your observed puddle. Have you considered these? – Lelouch Jul 25 '16 at 03:48
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    It is highly commendable that you are pursuing your curiosity through experiments. The problem which I see here is that we have no details about the experiments which you have performed, so in trying to explain why they don't agree with theory we are "shooting in the dark." For example : Did you give the honey sufficient time to spread out? Is there any consistency in the results you have obtained? – sammy gerbil Jul 25 '16 at 13:31
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    I would try various oils. They often have very well-characterised properties as engineers care a lot about this, and are not very variable for the same reason. –  Jul 25 '16 at 20:25
  • The main problems with the experiments is finding the viscosity and all the other variables, some of them are not easy to find in common liquids and the ones well described are usually hard to find in places other than a lab. – Rafa Jul 25 '16 at 22:11
  • I presume you've used dimensional analysis to get your formula, and have chosen the value of $\kappa$ to provide a good fit to your data. Viscosity probably shouldn't be included, because it is a static problem. Have you really obtained values for $\eta, \sigma, \rho$ for each fluid? Or have you made some guesses? How are you measuring H? ... What happens if you use the "famous" formula with a constant parameter? – sammy gerbil Jul 26 '16 at 00:36
  • I am rather sceptical about your result for honey, which is a mixture of varying composition and properties. Viscosity in particular is highly sensitive to temperature... Where did you get your formula for contact angle from? – sammy gerbil Jul 26 '16 at 00:43
  • The reason I used viscosity for the height is that, even thought it's a statics problem, when you're pouring the liquid, I thought that maybe viscosity may come in play in stopping the liquid from spilling further into the ground. The value of $\kappa$ was obtained graphically like this: When you use dimensional analysis, you get H up to a constant K and as a function of an exponent of one of the variables. – Rafa Jul 26 '16 at 01:08
  • Using the values obtained in the experiment for water and honey, you can get 2 curves that intersect in a point where the x value of this point is the value of the constant, and the y value serves to get the value of the exponent. Which approximately gives the formula above and works quite well with the other two liquids. About the contact angle formula, it's obtained using H(r) and imposing that the volume obtained by integrating is equal to the volume you've poured (which you know by using a syringe for example). If anyone wants it I'll edit the whole deduction in (it's rather long). – Rafa Jul 26 '16 at 01:09
  • I have voted to leave this question open because it is (in essence) no different from this popular one : Why is the vibration in my wire acting oddly? http://physics.stackexchange.com/q/267208 – sammy gerbil Jul 26 '16 at 15:17
  • Related : What is the maximum height for a puddle of water, assuming stp? http://physics.stackexchange.com/q/144269. Also very interesting is "Understanding a Spreading Puddle" at http://physics.aps.org/articles/v8/69 - this says that classical fluid theory cannot explain why puddles stop spreading. – sammy gerbil Jul 26 '16 at 16:21

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