11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action
2 min read•june 18, 2024
Liquids exhibit fascinating behaviors due to cohesive and adhesive forces. These molecular attractions create , allowing water striders to walk on ponds and raindrops to form. They also enable , which helps plants transport water and makes paper towels absorbent.
Understanding these liquid properties is crucial in physics and has wide-ranging applications. From designing better ink pens to developing devices for medical diagnostics, and in liquids play a vital role in our daily lives and technological advancements.
Cohesion and Adhesion in Liquids
Cohesive vs adhesive forces
attract molecules of the same substance hold liquid molecules together (water molecules in a drop)
Stronger cohesive forces create higher liquids bead up on surfaces
Adhesive forces attract molecules of different substances occur between liquid and container molecules (water adhering to glass)
Adhesive forces cause liquids to wet surfaces spread out on contact
play a crucial role in determining the strength of cohesive and adhesive forces
Surface tension in liquids
Property of liquids caused by cohesive forces molecules at surface experience unbalanced forces
Acts like an elastic membrane minimizes liquid's surface area (water strider walking on pond)
Allows formation of droplets (rain) and bubbles (soap)
Enables floating of denser objects (paperclip on water surface)
Affected by temperature higher temperatures reduce surface tension
Impurities like (dish soap) can lower surface tension
occurs at the boundary between two immiscible liquids
Capillary action and applications
Ability of liquid to flow through narrow spaces without external forces (water rising in a straw)
Occurs when adhesive forces between liquid and surface exceed cohesive forces within liquid
Liquid drawn upwards against gravity narrower spaces enable higher rise
Height of liquid column determined by:
h=ρgr2γcosθ
γ: surface tension
θ:
ρ: liquid density
g: acceleration due to gravity
r: radius of the tube
describes the relationship between capillary rise and tube radius
Enables water transport in plants through (trees)
Absorption of water by soil and roots (crops)
in fabrics (towels) and paper (paper towels)
Ink flow in pens (fountain pens) and printing (inkjet)
Microfluidic devices for chemical analysis (lab-on-a-chip) and medical diagnostics (blood tests)
Fluid properties and behavior
measures a fluid's resistance to flow and deformation
studies the motion of liquids and gases under various forces
These properties influence and surface tension phenomena
Key Terms to Review (28)
Adhesion: Adhesion is the phenomenon where two different surfaces or materials are held together by intermolecular forces, without the involvement of any intervening liquid. It is a fundamental concept that underpins various physical and biological processes, from the ability of geckos to climb walls to the cohesion of liquids and the function of biological membranes.
Capillary action: Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of external forces. It occurs due to the combination of cohesive forces within the liquid and adhesive forces between the liquid and surrounding surfaces.
Capillary Action: Capillary action, also known as capillarity, is the ability of a liquid to flow through narrow spaces without the assistance of, and sometimes against, external forces like gravity. This phenomenon is driven by the intermolecular attractive forces between the liquid and the surrounding solid surfaces, and is a key concept in understanding the behavior of liquids in small-scale systems.
Cohesion: Cohesion is the intermolecular force that causes like molecules to stick together, which is particularly significant in liquids. This property is crucial for understanding how fluids behave, including their ability to maintain shape and resist external forces. Cohesion plays a vital role in phenomena such as surface tension, where the cohesive forces at the surface of a liquid create a 'skin' effect, and is also essential in biological systems where it influences processes such as water transport in plants.
Cohesive forces: Cohesive forces are the intermolecular attractions that hold molecules of the same substance together. These forces play a critical role in phenomena such as surface tension and capillary action.
Contact angle: The contact angle is the angle formed between a liquid droplet and a solid surface. It quantifies the wettability of the solid surface by the liquid.
Contact Angle: The contact angle is the angle formed between the surface of a liquid and the surface of the solid it is in contact with. It is a measure of the wettability of a solid surface by a liquid, and is an important factor in understanding surface tension and capillary action in liquids.
Du Noüy ring: The Du Noüy ring is a method used to measure the surface tension of liquids by determining the force required to detach a ring from the surface of a liquid. This technique highlights the balance between cohesive forces among liquid molecules and adhesive forces between the liquid and the ring, demonstrating key principles of cohesion and adhesion in liquids. The results obtained from this method provide valuable insights into surface tension, which is crucial for understanding phenomena like capillary action and liquid behavior.
Dynes per Centimeter: Dynes per centimeter is the unit used to measure surface tension, which is the force per unit length acting on the surface of a liquid. It quantifies the cohesive forces between the molecules at the liquid's surface, which give rise to phenomena like surface tension and capillary action.
Fluid dynamics: Fluid dynamics is the branch of physics that studies the behavior of fluids (liquids and gases) in motion. It examines how forces affect the flow and movement of these substances, encompassing concepts like pressure, velocity, and viscosity, which are crucial in understanding phenomena in both natural and engineered systems.
Hydrogen Bonding: Hydrogen bonding is a special type of dipole-dipole attraction that occurs between a hydrogen atom covalently bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine, and another nearby highly electronegative atom. This intermolecular force is responsible for many of the unique properties of water and other hydrogen-containing compounds.
Interfacial Tension: Interfacial tension is a measure of the cohesive force that exists at the boundary or interface between two immiscible fluids, such as a liquid and a gas or two different liquids. It arises due to the imbalance of intermolecular attractive forces at the interface, which causes the surface to behave like an elastic sheet.
Intermolecular Forces: Intermolecular forces are the attractive or repulsive forces that exist between molecules, which determine the physical and chemical properties of substances. These forces play a crucial role in understanding the behavior of fluids, the surface tension and capillary action of liquids, as well as the phase changes that occur in matter.
Jurin's Law: Jurin's Law describes the relationship between the height a liquid rises in a capillary tube and the radius of that tube, showing how liquids move through narrow spaces due to surface tension. This principle highlights the balance between cohesive forces within the liquid and adhesive forces between the liquid and the walls of the tube. The law is essential for understanding phenomena like capillary action, which is critical in various natural and biological processes.
Meniscus: The meniscus is the curved upper surface of a liquid in a container, caused by the interaction between the liquid and the container's walls. This phenomenon is a result of the cohesive and adhesive forces acting on the liquid molecules.
Microfluidic: Microfluidics is the study and application of systems that manipulate and control small (10-9 to 10-18 liters) amounts of fluids, utilizing channels with dimensions ranging from tens to hundreds of micrometers. This field intersects with the topics of cohesion, adhesion, surface tension, and capillary action in liquids.
Surface Energy: Surface energy is the amount of energy required to create a new unit area of surface in a material. It is a measure of the cohesive forces that hold the surface atoms of a liquid or solid together, and it plays a crucial role in phenomena such as surface tension and capillary action.
Surface tension: Surface tension is the cohesive force at the surface of a liquid that makes it behave as an elastic sheet. It results from the greater attraction of liquid molecules to each other than to the molecules in the air.
Surface Tension: Surface tension is a property of liquids that arises from the cohesive forces between the molecules at the liquid's surface, causing the surface to behave like an elastic sheet. This property is crucial in understanding the behavior of fluids and the phenomena of capillary action and wetting.
Surfactants: Surfactants are compounds that lower the surface tension of liquids, allowing them to spread and penetrate more easily. They are commonly used in various applications, including detergents, emulsifiers, and wetting agents, and play a crucial role in the context of cohesion, adhesion, surface tension, and capillary action in liquids.
Thomas Young: Thomas Young was a renowned British polymath who made significant contributions to the fields of optics, wave theory, and surface tension. His work laid the foundations for our understanding of the wave nature of light and the principles of interference, diffraction, and thin-film interference.
Van der Waals Forces: van der Waals forces are weak intermolecular attractive forces that arise between neutral atoms or molecules. These forces, named after the Dutch physicist Johannes van der Waals, play a crucial role in the cohesion and adhesion of liquids, as well as in the surface tension and capillary action observed in many systems.
Viscosity: Viscosity is a measure of a fluid's resistance to deformation or flow. It quantifies the internal friction within the fluid when it is in motion.
Viscosity: Viscosity is a measure of the resistance of a fluid to flow. It describes the internal friction within a fluid that causes it to resist motion and flow. Viscosity is a crucial property that affects the behavior of fluids in various contexts, including fluid dynamics, heat transfer, and transport processes.
Wetting: Wetting refers to the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions between the liquid and the solid. This process involves adhesive forces between the liquid molecules and the solid surface being stronger than the cohesive forces among the liquid molecules themselves. The degree of wetting can influence various phenomena, including how liquids spread on surfaces and their behavior in capillary action.
Wicking: Wicking is the ability of a liquid to flow through a porous material or narrow spaces without the assistance of external forces, such as gravity. It is a key concept in understanding the behavior of liquids in the context of cohesion, adhesion, surface tension, and capillary action.
Xylem Vessels: Xylem vessels are specialized plant cells that form long, continuous tubes responsible for the upward transport of water and dissolved nutrients from the roots to the leaves. They are a crucial component of the plant's vascular system, playing a vital role in the processes of cohesion and adhesion in liquids, as well as surface tension and capillary action.
Young-Laplace Equation: The Young-Laplace equation is a fundamental relationship that describes the pressure difference across a curved surface, such as the interface between a liquid and a gas or between two immiscible liquids. It is a crucial concept in understanding surface tension and capillary action in liquids.