week 7 Flashcards
cohesion
the attractive forces between substances that are alike
adhesion
the attractive forces between unlike substances
cohesive forces are responsible for
surface tension
surface tension
a property of liquid surfaces resulting from intermolecular bonding which causes the liquid to minimize its surface & resist its deformation
surface tension acts to
reduce the surface area of a body of liquid
the size of the surface tension can be measured by determining
the force required to hold in place a wire, being used to stretch a film of liquid
surface tension equation
γ = F / L
we usually multiply the length by 2 when solving for surface tension
the length over which the force is being applied in the diagram shown is twice the length of the wire, as the liquid film has two surfaces
surface tension γ has units
N . m^-1
in order to prevent surface tension from collapsing a bubble…
the pressure inside the bubble must be greater than the outside pressure
when two bubbles collide
air always flows from the smaller to the larger bubble
(this is because pressure is greater in the smaller bubble)
to calculate gauge pressure
ΔP = 4γ / r
surfactant
a substance that, when added to a liquid, reduces the liquid’s surface tension
surfactant molecules tend to
concentrate near the surface
what makes a substance a surfactant?
having one hydrophilic and one hydrophobic side
surfactants are of major importance to
lung function
interfacial tension
the tension at the interface (a point where 2 systems meet/interact)
what determines interfacial tension?
the size of the adhesive forces
(usually between the liquid and walls of the container)
what are two types of interfacial tension?
- beading
- wetting
water beading an oily/waxy surface
- the cohesive intermolecular forces between water molecules are larger than the adhesive forces between water molecules and molecules on the surface
- this results in a compact droplet with a large contact angle, θ.
water wetting a hydrophilic surface (ex: glass)
- the adhesive forces between the water and the surface are stronger than the cohesive forces between the water molecules
- the droplet spreads out, and the contact angle is low
_____ ‘wets’ the surface of glass, while _____ does not ‘wet’ the surface of glass
water, mercury
contact angle
- a quantitative measure of the tendency to bead
- the angle that the edge of the liquid-air surface makes with the liquid-solid surface
contact angles more than 90˚are indicative of
beading
contact angles less than 90˚are indicative of
wetting
if the cohesive forces are weak in comparison to the adhesive forces for a fluid in a vertical tube, the contact angle will be _____ giving a _____ meniscus
small, negative
if the cohesive forces are strong in comparison to the adhesive forces for a fluid in a vertical tube, the contact angle will be _____ giving a _____ meniscus
large, positive
capillary action
when a thin glass tube is placed in a liquid such as water, the liquid often rises up the tube
why does more liquid rise the smaller the tube is?
a smaller radius means more contact with the surface for a particular volume of liquid, and hence a greater mass of liquid that can be supported by the contact force
h = 2𝛾𝑐𝑜𝑠𝜃 / 𝜌𝑔𝑟
h - height the liquid travels up the tube above the level of surrounding liquid
γ - surface tension
𝜃 - contact angle (angle liquid surface makes with tube surface)
𝜌 - density
r - tube radius
g - acceleration due to gravity
