Surface Tension and Energy Flashcards
Surface Tension
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- Contracting force per unit length along perimeter of a surface
- Work required to create a new unit area of a liquid
- Specific surface energy - excess surface free energy per
unit area - Restoring force to resist increase in area
γ = force/length or surface energy/area (i.e., ΔW/ΔA)
Slider and Floating Needle
Moving slider does work to create new surface
W = Fslider.∆l = 2.γ.L.∆l = γ.∆A (where ∆A = 2.l.∆l)
Capillary Action
Water-glass: adh>coh, wetting, 0ᵒ < θ < 90ᵒ
- Thermo: Adhesive force vertical component = weight
- F=γ*circumf=γ*2pi*r
- m=p/V
- V=pi*r^2*h
- Mechanics: Pressure lower in liquid than gas phase
- ΔP = 2γ/R=(2γcosθ)/r
- ΔP must equal the hydrostatic pressure drop in capillary = ρΔhg = (2γcosθ)/r
Mercury-glass:coh>adh, dry, 90ᵒ < θ < 180ᵒ
Pressure inside drop of liquid
Pi - Po = 2γ / R
Surface tension contracts the balloon. To counteract this tendency, the balloon has a greater interior air pressure acting to expand the balloon.
Spreading coefficient
Young’s Equation
Can’t measure γSG or γSL experimentally - need multiple liquid probe to estimate (Zisman).
Assumptions
- Solid surface is rigid and non-deformable
- Solid surface is immobile and cannot reorient in
response to liquid probe - Solid surface is typically smooth (tested with AFM)
- Homogeneous and uniform
- Liquid phase is known and remains constant
- Liquid vapor does not adsorb on the solid surface to
change free energy - gass bubbles?
Susceptible to
- Roughness
- Wetting or de-wetting amplified
- Increases SA, increases tension of surface in adhesion forces, increases wettability, decreases angle
- Contamination
- Grease film presence will increase angle and hydrophobicity
- Hysteresis
- Heterogeneities and rearrangments
- Uneven spreading of liquid
- Anisotropy and higher energy directions
Zisman Method
Critical surface tension (γC) for a solid
- Zisman Plot: Plot contact angles (θ or cos θ) against surface tensions of the probe liquids
- γLV = Linear plot
- Extrapolate line to θ=0ᵒ (i.e., complete spreading)
- Critical surface tension (γc) of the solid surface is equal to the liquid surface tension where θ = 0ᵒ (i.e., cos θ = 1).
Finding γSL given assumption that γSG=γLG, the liquid and solid have interfacial tension γSL=0 (polymer solid, liquid saturated hydrocarbons)
- This is principle of critical surface tension of wetting
γc with Bio-response
Wenzel Eq
On rough surgfaces
- angle on rough surface increases if hydrophobic
- angle on rough surfaces decreases if hydrophilic
amplification
Can be done through sandblasting, measured by AFM
Cassi-Baxter Eq
Accounts for surfaces made of two different materials (chemically heterogeneous - air pockets formed by roughness)
- if a water droplet does not entirely wet the rough surface and leaves pockets of air between the droplet and the substrate, then the observed contact angle is influenced by the fraction f of the droplet that is actually in contact with the surface
f2 is fraction in air, f1 fraction in contact with surface, theta-1 is observed angle
Plasma treatment
Plasma treatment
- Increas surface energy
- Gas (O2 or N2) disrupts molecules and creates a free radical layer
- Particles implanted on surface to promote reactivity
Glass Functionalization (silanization)
- Surface reaction yields strong – Si-O-Si bonds and
ethanol in this case - Amine groups are now available on the glass surface
to undergo additional reactions - Can modify the surface with higher/lower energy
groups
SAMs
Hydrophilic head group, H-C tail and function group R
Can be switchable and change structure with temperature
Young-Laplace Pressure
Relates the pressure difference (ΔP ) across the surface of a liquid to the surface curvature and liquid surface tension(γ) for the radius of curvature in directions 1 (R1) and 2 (R2).
ΔP = γ (1/R1 + 1/R2)
Surface tension balances the outward force due to wall pressure difference, or the surface tension tends to compress the droplet, increasing the internal P
Cylinder
R2 is infinity and the 2nd term goes to 0 as cylinder is thin
Cylinder Radius
Laplace derivation
Sphere
Air bubble in water
Capillarity force balance
Equation for any curved surface
Aneurysms
Needle sessile drop for contact angle
Dynamic sessile drop:
- determines the largest contact angle possible (advancing angle) without increasing its solid/liquid interfacial area by adding volume dynamically.
- Volume is removed to produce the smallest possible angle, the receding angle.
- The difference is the contact angle hysteresis
- The method is used especially with super hydrophobic samples, as this is the only practical method for such a high contact angle substrate.
- Multiple droplets can be deposited in various locations on the sample to determine heterogeneity
Tilting for Contact Angle
Tilt technique:
- A drop is dispensed on a level surface, and the surface is then tilted from 0° to 90°.
- As the tilt increases the downhill contact angle will increase and represents the advancing contact angle while the uphill side will decrease; this is the receding contact angle.
- The advancing and receding angles are measured at the point at which the droplet starts to move on the substrate.
- At the same point, the roll-off angle can be defined.
- The method allows for the mapping of the substrate but is often criticized as the size of the drop can have an impact on the measured values.
