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50

The mechanism at play here is surface tension. The cohesion of the molecules of water is what keeps the wasp afloat. Due to this cohesion, the surface of the water behaves like a membrane and is curved inwards. The light rays that would be refracted from the perfectly flat surface are now incident at an altered angle and are reflected or refracted by ...


22

It's a combination of two effects: buoyancy and adhesion. Buoyancy lifts the cork up as much as possible, until it displaces its own weight of water (Archimedes' principle). For this reason, the cork will seek the highest point of the water level. Because of adhesion between the water molecules and the glass, the water level is highest at the edges (the ...


19

What seems to be happening is that capillary effects in the presence of gravity create a situation in which the cork being maximally decentralized in the glass corresponds to a minimum energy configuration. My guess is that the cork is non-wetting, and therefore surrounded by a water surface that bends down in the proximity of the cork, thereby creating a ...


17

There is a great paper from the group of Howard Stone on this subject: Wetting of flexible fibre arrays (freely available here, but for some reason I am not allowed to link to it normally: http://211.144.68.84:9998/91keshi/Public/File/34/482-7386/pdf/nature10779.pdf) They specifically study when 2 closely positioned parallel fibers (i.e. hairs) clump ...


14

That is a truly amazing picture! I am by no means an expert, but I have an idea. When the wasp stands on the water, it is curved down slightly. The light that hits these parts will then be bent more outwards than if it just hit regular water. This happens at every side of the circle, so the light is always bent out, and doesn't reach the bottom at those ...


13

I looked up the answer to this one in a book published in 1914 - you don't get many citations 99 years old! For the interested, the book is "A Textbook of Physics Vol 1" by J. H. Poynting and J. J. Thompson, page 188 in my copy. Incidentally that's the same J. J. Thompson who discovered the electron - Poynting has a vector named after him though only ...


8

This is a great example of how nice it often is to reason about refraction stuff using Fermat's principle. Let's reduce all this to 2 dimensions. The surface tension produces something like this: Now if we want to now were a light "ray" needs to go to get from some light source, we just need to find the way that takes it the least time. Light is slower in ...


7

No. Free-floating bubbles form essentially spherical shapes. In general, they form shapes that minimize surface area, subject to constraints such as the bubble having to enclose a fixed volume. One way to see why the bowl shape you imagine wouldn't work is to consider a small element of the surface right near the "corner" of the bowl (where the spherical ...


7

First of all, Marek is right that a surface tension exists only between two different materials (well, I would say between two different phases - for example water and ice). So let's rephrase the question as "Are there two phases with zero surface tension?" and elaborate a little on the answer. The surface tension is the excess free energy (technically the ...


6

A drop that is free falling in vacuum is spherical. This is because free falling in a gravitational field is the same thing as being at rest with no gravitational field present: the gravitational field and the acceleration cancel each other out. Rain drops falling to the earth can have various shapes depending on their size, although I am not aware that ...


6

Surface tension is not a property of materials but of interfaces between two (or more) materials. It is implicit in its definition that the interface separates two kinds of materials that behave differently (otherwise the interface would be just some imaginary surface inside the one material with no physical meaning) and so there must always be some surface ...


6

The article's preprint Mayer H. C., Krechetnikov R. "Walking with coffee: why does it spill?," Phys. Rev. E 85, 046117 (2012). is available from the UCSB site. From a glance of the article the phenomenon is not specific only to coffee. The authors make use of the next formula: The natural frequencies of oscillations of a frictionless, ...


6

Liquids are a state of matter in which the atoms or molecules are held together by chemical bonds (a difference from gases) but the bonds are weak enough for the shape to be variable (a difference from solids). That's why it is not possible to increase or decrease their volume much; the amount of energy from these chemical bonds would rapidly increase ...


6

The highest pressure in the ocean is at the bottom of the Mariana trench, where the pressure is 1,086 atmospheres. Using the online calculator for the properties on nitrogen at 4°C and 1,000 atmospheres the density comes out as 602 kg/m$^3$, which is still less than water. So a bubble of nitrogen would rise even at the deepest point in the ocean. Response ...


6

The real issue is that the cup wasn't really full so that adding anything more would make it spill. You can clearly see the the level slowly growing above the top of the cup, as would be expected due to surface tension. Eventually another coin finally exceeded the limit, and a little water spilled. There is really nothing extraordinary going on here. ...


5

The problem is azimuthally-symmetric, so the tangential direction is the one with no azimuthal component, i.e. the one "straight up the side" if you were a small mountain climber climbing from the surface to the top of the drop. All three surface tensions are required because all three exert forces. For example, if the solid-air surface tension were ...


5

A bubble, while it still exists, is balanced by three factors: 1) Surface Tension of the soapy water. 2) Internal Pressure applied by the air inside the bubble on the surface. 3) Atmospheric Pressure. When any of these are imbalanced, one force is greater than the others and this causes the bubble to pop. If you're talking about why do they burst in ...


5

In the west the vast majority of towels are made from cotton, and cotton is basically cellulose. The surface of cellulose is fairly reactive (the bulk isn't unless you're a termite!) and will react with water to produce surface hydroxyl groups and negatively charged groups. Both of these lower the contact angle of water on the fibres and hence increase ...


5

Indeed, the second term is the potential gravitational energy. Gravitational energy for a continuous body could be calculated as $$ E_g = \int_\text{body} \rho g\, z \,d^3x, $$ where we assume that gravitational acceleration $g$ has only $z$-component. The 'body' you have in your problem would stretch to length $L$ along the $y$ axis and lays between the ...


5

The formula for capillary rise that most people know is easily derived through a pressure balance between the capillary pressure and the hydrostatic pressure. The hydrostatic pressure equals $$\Delta P_h=\rho g h$$ whereas the capillary pressure is $$\Delta P_c=\frac{2\gamma}{R}=\frac{2\gamma \cos \theta}{r}$$ So balancing these we get our 'famous' ...


5

As I commented, I would think that any 3D hydrodynamics code would work. The basics of hydrodynamics can be summed up in the following five equations: \begin{eqnarray} \frac{\partial \rho}{\partial t}+\nabla\cdot\rho\mathbf{v}=0 \tag{1} \\ \frac{\partial \rho\mathbf{v}}{\partial t}+\nabla\cdot\left[\rho\mathbf{v}\otimes\mathbf{v}+P\mathbb ...


5

Unless I have made a conceptual mistake (which is very possible), surface tension plays essentially no role in the damping of the impact of a fast-moving object with a liquid surface. To see this, a simple way to model it is to pretend that the water isn't there, but only its surface is, and see what happens when an object deforms this surface. Let there ...


4

There is no need to ask the question about a half-sphere because a general answer regarding any surface edge of a substance with the desired properties (I'll call "bubble surface") will apply for that case. This is a problem about surface tension (ST). Of course, there are cases where a non-enclosed bubble surface exists, for instance in the case of a ...


4

To see if a process will take place you need to calculate it's Gibbs free energy $\Delta G$. This is defined as: $$\Delta G = \Delta H - T\Delta S$$ The quantity $\Delta H$ is the Helmholtz free energy and for liquids and solids is roughly the amount of energy released ($\Delta H$ is negative if energy is released and positive if energy is absorbed). ...


4

What you quote is basically the definition of the Rayleigh-Taylor instability. See, e.g. this paper in Physica D. Rayleigh-Taylor instability refers precisely to the linear instability of the interface between two fluids when a less dense fluid is pushing on a denser one. For details of the linear stability analysis you can see Wikipedia or most textbooks on ...


4

It's caused by surface tension of the air/water interface. As the jet starts to pinch off it creates a neck between the jet and the developing drop, and this neck stretches the surface. When the drop breaks free, the stretched region rebounds. This creates a splash that ejects the small droplets from the trailing edge of the drop and the leading edge of the ...


4

The wicking is not occuring because of a siphon action. Rather, capillary action is responsible. The abrupt stop at the edge of the cup has two likely explanations; one -- all your solvent evaporated before capillary action wicked solute that far, or two -- The cup preserves a higher relative humidity within, which drops abruptly outside the cup, thus ...


4

This answer is a bit of a long story, but I have split it up for the different statements for your convenience. Having thought about it a bit more after the discussion with @Mephisto I actually believe that Bernoulli's equation is not applicable in points B and C, because it is based on conservation of energy and therefore only applies if wall friction is ...


4

The German-language Wikipedia's page on C.F. Gauß lists a German translation of this work by Rudolf H. Weber, titled Allgemeine Grundlagen einer Theorie der Gestalt von Flüssigkeiten im Zustand des Gleichgewichts (publication date: 1903), available on the Internet Archive (abstract, pdf).



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