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

Un ago d'acciaio o una lama di rasoio possono galleggiare sull'acqua se posizionati delicatamente a causa della tensione superficiale, che si verifica all'interfaccia tra un liquido e un gas o due liquidi che non si mescolano.
La tensione superficiale fa sì che la superficie del liquido si comporti come una membrana elastica tesa, sostenendo così l'oggetto.
La tensione superficiale è il risultato delle forze coesive che agiscono tra le molecole liquide. Le molecole all'interno del liquido sono circondate uniformemente da altre molecole, con conseguente bilanciamento delle forze in tutte le direzioni.
D'altra parte, le molecole in superficie subiscono una forza sbilanciata verso l'interno del liquido, creando un effetto pelle sulla superficie.
L'intensità di queste forze coesive per unità di lunghezza è chiamata tensione superficiale. La tensione superficiale varia con la temperatura e il tipo di fluido, diminuendo all'aumentare della temperatura.
La tensione superficiale è fondamentale in processi come la formazione di goccioline. Nei sistemi di irrigazione, l'acqua forma goccioline che possono essere distribuite uniformemente sulle piante.
Le forze coesive all'interno dell'acqua aiutano a mantenere la forma della goccia fino a quando non entra in contatto con una superficie, garantendo un utilizzo e una distribuzione efficienti dell'acqua.

tensione superficiale di un fluido

La tensione superficiale è una proprietà fondamentale dei fluidi, è un fenomeno che si verifica al confine tra un liquido e un gas o tra due liquidi immiscibili. Deriva dalle forze coesive tra le molecole sulla superficie del fluido, e crea un effetto simile a una membrana elastica allungata. All'interno di ogni fluido, le molecole sono attratte equamente in tutte le direzioni dalle molecole vicine, ma le molecole di superficie, subiscono una forza netta verso l'interno, con conseguente determinarsi della tensione superficiale.

La tensione superficiale varia in funzione del tipo di fluido e della sua relativa temperatura, in genere diminuendo all'aumentare di quest’ultima. Temperature più elevate riducono la coesione molecolare, abbassando così la tensione superficiale. Questa proprietà è fondamentale per spiegare la formazione di goccioline, bolle e il comportamento dei getti di liquido. Ad esempio, le goccioline d'acqua, creano forme sferiche per ridurre al minimo l'area superficiale, suggerendo il ruolo della tensione superficiale nella riduzione dell'energia.

La capillarità, un altro effetto della tensione superficiale, si osserva quando un liquido si muove verso l'alto o verso il basso in un tubo sottile, rispetto al livello del liquido circostante. Il diametro del tubo influenza l'altezza che il liquido raggiunge nel tubo; tubi più stretti determinano rialzi più alti.

La tensione superficiale inoltre, consente a oggetti leggeri di galleggiare su una superficie liquida. Ad esempio, un ago d'acciaio può galleggiare sull'acqua, se posizionato delicatamente, poiché sostenuto dalla tensione superficiale. Allo stesso modo, insetti come i gerridi, sfruttano la tensione superficiale per camminare sull'acqua senza affondare.

Tensione Superficiale di un Fluido

Un ago d'acciaio o una lama di rasoio possono galleggiare sull'acqua se posizionati delicatamente a causa della tensione superficiale, che si verifica all'interfaccia tra un liquido e un gas o ...

Education

Didattica

JoVE Core

Civil Engineering

Ingegneria Civile

Unit 1

Unità 1

Fluid Properties

Proprietà dei fluidi

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

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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

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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

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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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