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

Une aiguille en acier ou une lame de rasoir peut flotter sur l’eau si elle est placée doucement en raison de la tension superficielle, qui se produit à l’interface entre un liquide et un gaz ou deux liquides qui ne se mélangent pas.
La tension superficielle fait que la surface du liquide se comporte comme une membrane élastique étirée, soutenant ainsi l’objet.
La tension superficielle résulte des forces de cohésion agissant entre les molécules liquides. Les molécules à l’intérieur du liquide sont entourées uniformément par d’autres molécules, ce qui entraîne des forces équilibrées dans toutes les directions.
D'autre part, les molécules à la surface subissent une force déséquilibrée vers l'intérieur du liquide, créant un effet de peau à la surface.
L’intensité de ces forces de cohésion par unité de longueur est appelée tension superficielle. La tension superficielle varie en fonction de la température et du type de fluide, diminuant à mesure que la température augmente.
La tension superficielle est cruciale dans des processus tels que la formation de gouttelettes. Dans les systèmes d’irrigation, l’eau forme des gouttelettes qui peuvent être réparties uniformément sur les plantes.
Les forces de cohésion à l’intérieur de l’eau aident à maintenir la forme des gouttelettes jusqu’à ce qu’elles entrent en contact avec une surface, assurant ainsi une utilisation et une distribution efficaces de l’eau.

tension superficielle d'un fluide

La tension superficielle est une propriété fondamentale des fluides, qui se produit à la frontière entre un liquide et un gaz ou entre deux liquides non miscibles. Ce phénomène résulte des forces de cohésion entre les molécules à la surface du fluide, créant un effet similaire à celui d'une membrane élastique étirée. À l'intérieur de chaque fluide, les molécules sont attirées de manière égale dans toutes les directions par les molécules voisines, mais les molécules de surface subissent une force nette vers l'intérieur, ce qui entraîne une tension superficielle.

La tension superficielle varie en fonction du type de fluide et de sa température, diminuant généralement à mesure que la température augmente. Des températures plus élevées réduisent la cohésion moléculaire, diminuant ainsi la tension superficielle. Cette propriété est cruciale pour expliquer la formation de gouttelettes, de bulles et le comportement des jets de liquide. Par exemple, les gouttelettes d'eau forment des formes sphériques pour minimiser la surface, illustrant le rôle de la tension superficielle dans la réduction de l'énergie.

L'action capillaire, un autre effet de la tension superficielle, est observée lorsqu'un liquide monte ou descend dans un tube fin par rapport au niveau de liquide environnant. Le diamètre du tube influence la hauteur atteinte par le liquide dans le tube; Des tubes plus étroits entraînent des élévations plus élevées.

La tension superficielle permet également à des objets légers de flotter sur une surface liquide. Par exemple, une aiguille en acier peut flotter sur l'eau si elle est placée doucement, car la tension superficielle la soutient. De même, des insectes comme les araignées d'eau exploitent la tension superficielle pour marcher sur l'eau sans couler.

Tension superficielle d'un fluide

Une aiguille en acier ou une lame de rasoir peut flotter sur l’eau si elle est placée doucement en raison de la tension superficielle, qui se produit à l’interface entre un liquide et un gaz ou ...

Education

Enseignement

JoVE Core

Civil Engineering

Génie civil

Unit 1

Unité 1

Fluid Properties

Propriétés des fluides

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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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