What is the reason?
Is it caused by their narrow shape, the soft material, walking vibration or something else?
[Physics] Why do earphone wires always get tangled up in pocket
everyday-lifeexperimental-physicsstring
Related Solutions
Earbud cords form coils in your pocket. Even if you crumple the cord, it eventually finds its way into coils as you move and the cord is agitated. The coils lie in planes that are stacked.
It's been found that agitated coils spontaneously braid and knot themselves. See this paper: http://www.pnas.org/content/104/42/16432.full, which found that formation of knots in an agitated string is dependent on (1) the agitation time, (2) the length of the string, (3) the type of agitation, and (3) the stiffness of the string. A long, flexible string tends toward theoretical predictions of knotting, whereas a stiff string resists self-knotting.
Flat earbud cords are extremely stiff on two of their four faces. They do not bend easily through the narrow faces. Only the wide faces can easily bend. The wide surfaces fold, but they resist bending in other directions, across their narrow sides. Also, they resist twisting, and will only twist in a corkscrew with a relatively wide radius. Think of a flat cord lying in a coil in the x,y plane. With its wide faces perpendicular to the plane, but parallel to the z axis, the flat cords exhibit stiffness along the z axis, and resistance to leaving the x,y plane. In order to self-braid, the cord would need to flex above and below the x,y plane. Their shape resists this.
Round cords, on the other hand, bend and twist in all directions, so there is little resistance to the loops of a coil flattening into ellipses of various major and minor axes which then shift lateral position and send bights above and below the planes of other ellipses. They have equal flexibility in all three planes, so their propensity to self-braid by sending loops above and below the x,y plane is uninhibited.
Some manufacturers have reduced self-braiding and self-knotting by stiffening their round earbud cords. But early models were notoriously flimsy, and braided with very little agitation.
There are not really any structural reasons for several layers of different softness/hardness. I believe that a hard surface as the outer layer is only added for a better user-feel.
If you only seek good protection, then you have two parameters to aim for:
- Make it soft and
- make it thick.
The softer and thicker the better. To prevent collision damage, you are looking at Newton's 2nd law:
$$\sum F= \frac{\Delta p}{\Delta t}$$
Your phone hits the floor and there will be a momentum-change $\Delta p$ from it's impact speed to zero. You can't reduce that. But you can change the duration $\Delta t$ of this impact. Larger collision duration means smaller forces $\sum F$ on the phone.
Increasing $\Delta t$ means that you must cover it in a material that absorbs the kinetic energy; materials that deform at collision. Therefore soft. Not too soft, because it should be able to absorb the entire amount of kinetic energy. By making a thicker layer of this soft material, you again acheive more energy absorbed. A pillow is perfect.
(Just remember that you don't want to add too much mass by adding this cover, since mass raises $\Delta p$. But luckily, lightweigth and softness usually follow along.)
So, there's not really any argument for more layers. The softest material able to absorb the whole amount of kinetic energy with the given thickness that you will allow for, is the prefered solution. Any other materials are merely for design or user-feel purposes.
As the PO points out in the comments, there may very well be an advantage in having a hard outer shell, because the energy then can be dispersed by a large area. Maybe we shouldn't rule out such outer shell completely then. We are entering the engineering field then, since the design of the shell and evenly distribution of soft material under evenly spaced outer material is important here.
Best Answer
Because they are too long for the confined space. Reasoning: A very short string can't clot (say 1mm). So it needs a certain length before it can start clotting.
If they could be placed neatly (say big pocket), they wouldn't clot because the ends are too far apart.
They must be flexible. A steel bar doesn't clot.
So you need certain elements to be just right:
In the usual case, the string will be stuffed into the pocket without much consideration, so it will actually start somewhat clotted. If the ends aren't fixed somewhere, they will easily weave with the rest of the string. What makes matters worse is that the ends aren't thin. So you can't untangle everything by simple pulling.
The easiest way to avoid clotting is to fixate the two ends. In my case, I have a clip to pin the volume control to my clothes. If you fixate the two ends with that clip, they don't clot.