Name: why water drops and needles float
Uploaded: 2025-02-13T15:43:08.000Z
Duration: 1 min 22 s
Channel: JoVE

鋼の針やカミソリの刃は、液体と気体、または混ざらない2つの液体との間の界面で発生する表面張力により、穏やかに置くと水に浮く可能性があります。
表面張力により、液体表面は伸びた弾性膜のように動作し、物体を支えます。
表面張力は、液体分子間に作用する凝集力から生じます。液体内の分子は他の分子に均一に囲まれているため、全方向にバランスの取れた力が生まれます。
一方、表面の分子は液体の内部に向かって不均衡な力を受け、表面に表皮効果を生み出します。
単位長さあたりのこれらの凝集力の強度は、表面張力と呼ばれます。表面張力は温度や流体の種類によって異なり、温度が上昇すると減少します。
表面張力は、液滴の形成などのプロセスで重要です。灌漑システムでは、水滴が形成され、植物全体に均一に分布することができます。
水中の凝集力は、水滴が表面に接触するまで液滴の形状を維持するのに役立ち、効率的な水の使用と分配を保証します。

流体の表面張力

表面張力は流体の基本的な特性で、液体と気体の境界、または混ざり合わない 2 つの液体の間に発生します。この現象は、流体の表面にある分子間の凝集力から生じ、伸縮性のある膜に似た効果を生み出します。各流体の内部では、分子は隣接する分子によってあらゆる方向に均等に引き付けられますが、表面の分子は正味の内向きの力を受けるため、表面張力が生じます。

 表面張力は流体の種類と温度によって異なり、通常は温度が上昇するにつれて低下します。温度が高くなると分子の凝集力が低下し、表面張力が低下します。この特性は、液滴、泡、および液体ジェットの挙動の形成を説明する上で非常に重要です。たとえば、液滴は表面積を最小化するために球形を形成します。これは、エネルギーを減らす表面張力の役割を示しています。

 毛細管現象は表面張力のもう 1 つの効果であり、液体が周囲の液体レベルと比較して細い管内を上下に移動する時に観察されます。管の直径は、管内で液体が到達する高さに影響します。管が細いほど、上昇幅が大きくなります。

 表面張力により、軽量の物体も液体の表面に浮かぶことができます。たとえば、表面張力によって支えられているため、鋼の針をそっと置くと水に浮かぶことができます。同様に、アメンボなどの昆虫は表面張力を利用して、沈むことなく水の上を歩くことができます。

鋼の針やかみそりの刃は、液体と気体の間の界面で発生する表面張力により、穏やかに置くと水に浮くことができます。

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Fluid Properties

流体の特性

KEY TERMS AND CONCEPTS

主要用語と概念

Surface Tension of Fluid

why water drops and needles float

Surface tension helps explain why water forms drops, supports small objects, and moves in thin tubes. It is a property of fluids at the boundary between a liquid and a gas, or between two immiscible liquids. At the surface, molecules are pulled inward by neighboring molecules. That inward pull makes the surface act a little like a stretched elastic membrane.

Inside a fluid, molecules are attracted in all directions. Surface molecules do not have the same balance of forces. They feel a net inward force, and that is what creates surface tension. The strength of this effect depends on the fluid, and it usually gets lower as temperature increases.

Higher temperature reduces molecular cohesion, which means the molecules hold together less strongly. Lower cohesion leads to lower surface tension. This change helps explain the behavior of droplets, bubbles, and liquid jets. Water droplets often become spherical because that shape minimizes surface area and lowers energy.

Surface tension also shows up in capillary action. In a thin tube, a liquid can rise above or fall below the level of the surrounding liquid. The tube diameter matters. Narrower tubes produce a higher rise in the liquid.

This same property can support light objects on water. A steel needle can float if it is placed gently on the surface. In the same way, water striders use surface tension to walk on water without sinking.

Why Water Drops and Needles Float

characteristics of fluids

Fluid Behavior in Liquids and Gases

Fluids include liquids and gases, and they behave differently from solids because of how their molecules are arranged. In solids, molecules are tightly packed and held by strong intermolecular forces. That structure helps solids keep their shape and resist deformation. Fluids have molecules that are farther apart and held by weaker forces. Because of this, they can flow and change shape easily. A fluid is any substance that deforms continuously when shearing stress is applied. Shearing stress...

density, specific weight, specific gravity and compressibility of fluid

density-specific-weight-specific-gravity-and-compressibility-of-fluid

Fluid Properties: Density, Weight and Compression

Density, specific weight, specific gravity, and compressibility are key fluid properties. Density is the mass per unit volume of a fluid. It helps describe how much mass a fluid system contains and affects buoyancy, pressure, flow behavior, viscosity, thermal conductivity, and sound propagation. In pipeline design, knowing density helps engineers make sure the pipe can support the fluid’s mass. Specific weight is the weight per unit volume. It is found by multiplying density by gravitational...

viscosity of fluid

How Temperature Changes Fluid Viscosity

Viscosity describes how much a fluid resists flow and deformation. It comes from internal friction between fluid layers that move past each other. Dynamic viscosity, shown by the Greek letter mu (μ), measures the force needed to move one layer over another. Newtonian fluids, such as water and air, have a linear relationship between shearing stress and the rate of shearing strain. Their viscosity stays constant, even when the applied stress changes. Non-Newtonian fluids, like ketchup,...

types of fluids

Newtonian vs Non-Newtonian Fluid Behavior

Fluids can be grouped by how they respond to shear stress. Shear stress is a force that acts across a material, and shear strain rate describes how quickly it changes shape or flows. In this lesson, fluids are divided into Newtonian and non-Newtonian types. Newtonian fluids follow Newton’s law of viscosity. Their shear stress changes in a linear way as the shear rate changes. That means their viscosity stays constant, so their flow is predictable and easier to analyze. Water, air, oil, and...

newtonian fluid: problem solving

Newtonian Fluid Viscosity and Shear Stress

Newtonian fluids have a constant viscosity, so their shear stress and shear strain rate stay directly proportional. This creates a linear, predictable link between force and flow. Water, air, and light oils are common examples of Newtonian fluids. A simple way to study this behavior is to place the fluid between two parallel plates. One plate stays still, and the other moves at a constant velocity. The fluid forms a velocity gradient, which is the change in speed across the distance between...

vapor pressure of fluid

Vapor Pressure and Boiling Behavior

Vapor pressure and boiling behavior are closely linked in fluids. Vapor pressure is the pressure a vapor exerts when it is in thermodynamic equilibrium with its liquid at a given temperature. It shows how strongly molecules tend to leave the liquid surface and move into the vapor phase. A closed container helps show how vapor pressure forms. If a liquid sits in a sealed space with a small air gap, and that space is evacuated, vapor molecules escape from the liquid into the empty space. The...

surface tension of fluid

Surface tension helps explain why water forms drops, supports small objects, and moves in thin tubes. It is a property of fluids at the boundary between a liquid and a gas, or between two immiscible liquids. At the surface, molecules are pulled inward by neighboring molecules. That inward pull makes the surface act a little like a stretched elastic membrane. Inside a fluid, molecules are attracted in all directions. Surface molecules do not have the same balance of forces. They feel a net...

capillarity in fluid

Capillary Action in Narrow Spaces

Capillary action is the movement of liquid through narrow spaces without any outside force. It depends on surface tension and adhesion between the liquid and nearby solid surfaces. This effect is common in narrow tubes, porous materials, and fine particles. Surface tension comes from cohesive forces between liquid molecules at the liquid-air boundary. These forces create a thin skin that resists outside pressure. When a capillary tube is placed in a liquid, the liquid level changes because of...

design example: deciding thickness of lubricating fluid in a shaft

design-example-deciding-thickness-of-lubricating-fluid-in-a-shaft

Lubricant Film Thickness for Shaft Bearings

Lubricant film thickness for shaft bearings is calculated to keep a rotating shaft from touching its bearing housing. A controlled layer of lubricant reduces friction, wear, and energy loss. It also helps the machine run smoothly by preventing metal-to-metal contact. The calculation starts with the shaft’s tangential velocity. First, the rotational speed is converted to angular velocity. Then that value is multiplied by the shaft radius. The result is the shaft’s linear speed at its surface...

key terms and concepts

Explore how surface tension shapes droplets, bubbles, capillary action, and floating needles. Learn why temperature and tube size change fluid behavior.

Why Water Drops and Needles Float | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

civil engineering

Why Water Drops and Needles Float

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