Water strider

Water Striders and Smaller Water Striders (Gerridae and Veliidae)... 

A molecule of a liquid attracts the molecules that surround it, and, in its turn, it is attracted by them (Figure 1.2). For the molecules that are inside a liquid, the resultant of all these forces is neutral, and all of them are in equilibrium by reacting with each other. When these molecules are on the surface, they are attracted by the molecules below and by the lateral ones, but not toward the outside. The resultant is a force directed inside the liquid. In its turn, the cohesion among the molecules supplies a force tangential to the surface. So, a fluid surface behaves like an elastic membrane that wraps and compresses the liquid below. The surface tension expresses the force with which the surface molecules attract each other. It is common observation that, due to the surface tension, it takes some effort for some bugs to climb out of the water in lakes. On the contrary, other insects, such as the marsh treaders and water striders, exploit the surface tension to skate on the water without sinking (Figure 1.3). 

Interactions such as surface tension also become more influential at smaller scales. Surface tension is due to attractions between molecules, such as electrical charges and hydrogen bonds, which cause them to bunch together and resist separation. For example, such forces create a tension on the surface of water because the molecules stick together, and this force is strong enough to let a bug known as the water strider walk on water. In Zetd s motor, the smaller drop grows imtil it reaches... 

Another familiar property of liquids is surface tension, the resistance of a liquid to spread out and increase its surface area. Surface tension is caused by the difference in intermolecular forces experienced by molecules at the surface of a liquid and those experienced by molecules in the interior. Molecules at the surface feel attractive forces on only one side and are thus drawn in toward the liquid, while molecules in the interior are surrounded and are drawn equally in all directions (Figure 10.8). The ability of a water strider to walk on water and the beading up of water on a newly waxed car are both due to surface tension. 

Surface tension allows a water strider to walk on a pond TABLE 10.6 Viscosities and Surface Tensions of Some Common Substances at 20°C ... 

If sand is moist, the slope of a sand pile can be higher. A sand castle can have vertical walls when it is built of moist sand in the morning, but as the afternoon wears on and the sand dries out, it cmmbles and collapses (mass wastes) until a stable slope forms. This is because the water makes the sand more cohesive. With the proper moisture content, there will be both water and air between most of the grains of sand. The boundary between the water and the air has surface tension— the same surface tension that supports water striders or pulls liquids up a capillary tube. In moist sand, surface tension holds the grains together like a weak cement. 

The surface tension of water supports this water strider. The nonpolar surfaces of its feet also help to repel the water. 

Q.l2.8 Water striders are insects that skate along the surface of ponds, streams, and other bodies of water. While eating a picnic next to a stream, you watch as somebody dumps a large amount of white powder upstream, as the cloudy water passes underneath the water striders, you notice the insects have trouble staying above the surface. Hypothesize what you think the white powder might be. 

A.12.8 One hypothesis is that the white powder is a surface active detergent. The detergent wouid iower tire surface tension making it difficuit for the water striders to stay on top of the water. 

Henderson pointed out another property of water its high surface tension, which is substantially higher than that of any other common liquid except mercury. This curious property allows water striders to walk across the surface of ponds or, as a parlor trick, partyers to float a double-edged razor blade in a bowl of water. More important, this property helps water to flow upward, against the force of gravity, in the tiny veins of even tall plants. 

Surface tension makes water act somewhat like an elastic film. Insects called water striders are able to walk on the filmlike surface of water. 

A FIGURE 11.18 Molecular-level view of surface tension. A water strider does not sink because of the high surfaoe tension of water. 

FIGURE 11.1 Surface tension, (a) The surface Is strengthened by intermolecular forces attracting surface molecules, (b) The water strider, a lightweight insect that does not provide enough force per unit area to break through the surface tension. Note that the strider does not walk on the sharp ends of its "toes." (c) With care, a paper clip can be placed so that it won t sink in the water, (d) On a dirty car, this wouldn t happen. The dirt would overcome the surface tension of the water droplets. 

The high surface tension of water keeps the water strider from sinking. 

Surface tension enables the water strider to "walk on water. 

Figure 4. SEM images of naturally superhydrophobic surfaces (a, b) lotus leaf and (c, d) water strider leg (reprinted with permission from WUey Interscience [29] and Nature Publishing Group [31]).

The Role of Adaptive-Deformation of Water Strider Leg in Its Walking on Water... 

Water strider leg, super load-bearing capability, superhydrophobicity, adaptive-deformation, explicit model... 

We used a hind leg that was cut off at its root from the body of a water strider captured in the Beijing area. It was adjusted to have different inihal dip angles cp measured from the water surface before its contact, as shown in Fig. 2(a-l)-2(a-4). The corresponding shapes of the deformed leg and the trodden puddles just before... 

Our comparative experiments reported in the present paper reveal that the water strider legs display an adaptive-deformation feature when pressing on a water surface and that this mechanism plays an important role in forming a deep puddle and in generating the superior load-bearing ability of the insect. Comparatively, the su-... 

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