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

강철 바늘이나 면도날은 액체와 기체 또는 혼합되지 않는 두 액체 사이의 계면에서 발생하는 표면 장력으로 인해 부드럽게 놓으면 물 위에 뜨 수 있습니다.
표면 장력은 액체 표면이 늘어난 탄성 멤브레인처럼 행동하도록 하여 물체를 지지합니다.
표면 장력은 액체 분자 사이에 작용하는 응집력으로 인해 발생합니다. 액체 내의 분자는 다른 분자에 의해 균일하게 둘러싸여 모든 방향으로 균형 잡힌 힘을 생성합니다.
반면에 표면의 분자는 액체 내부를 향해 불균형한 힘을 경험하여 표면에 피부 효과를 생성합니다.
단위 길이당 이러한 응집력의 강도를 표면 장력이라고 합니다. 표면 장력은 온도와 유체 유형에 따라 달라지며 온도가 증가함에 따라 감소합니다.
표면 장력은 액적 형성과 같은 과정에서 매우 중요합니다. 관개 시스템에서 물은 식물 전체에 고르게 분포될 수 있는 물방울을 형성합니다.
물 속의 응집력은 표면에 닿을 때까지 물방울 모양을 유지하는 데 도움이 되어 효율적인 물 사용 및 분배를 보장합니다.

유체의 표면 장력

표면 장력은 유체의 기본적인 속성으로, 액체와 기체 또는 두 개의 섞이지 않는 액체 사이의 경계에서 발생합니다. 이 현상은 유체 표면의 분자 간의 응집력에서 발생하여 늘어난 탄성 막과 유사한 효과를 생성합니다. 각 유체 내부에서 분자는 모든 방향으로 이웃 분자에 의해 동일하게 끌리지만 표면 분자는 순수한 안쪽 힘을 경험하여 표면 장력이 발생합니다.

표면 장력은 유체의 유형과 온도에 따라 달라지며 일반적으로 온도가 증가함에 따라 감소합니다. 온도가 높을수록 분자 응집력이 감소하여 표면 장력이 낮아집니다. 이 속성은 물방울, 거품의 형성 및 액체 제트의 동작을 설명하는 데 중요합니다. 예를 들어, 물방울은 표면적을 최소화하기 위해 구형을 형성하여 에너지를 줄이는 표면 장력의 역할을 보여줍니다.

표면 장력의 또 다른 효과인 모세관 현상은 액체가 주변 액체 수위에 비해 얇은 관을 위아래로 움직일 때 관찰됩니다. 관의 직경은 관에서 액체가 도달하는 높이에 영향을 미칩니다. 관이 좁을수록 더 높은 상승이 발생합니다.

표면 장력으로 인해 가벼운 물체도 액체 표면에 떠오를 수 있습니다. 예를 들어, 강철 바늘은 부드럽게 놓으면 표면 장력이 지지하기 때문에 물 위에 떠오를 수 있습니다. 마찬가지로 물거미와 같은 곤충은 표면 장력을 이용하여 가라앉지 않고 물 위를 걷습니다.

강철 바늘이나 면도날은 액체와 가스 또는 ...

Education

교육

JoVE Core

Civil Engineering

토목공학

Unit 1

제1단원

Fluid Properties

유체 특성

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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 offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

civil engineering

Why Water Drops and Needles Float

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