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
Soap bubble
Soap bubble has 2 liquid-air interfaces so twice the surface tension.
γ = P*r / 4
Surface tension measurement
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
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.
Capture bubble technique:
- The contact angle is measured between an air bubble of defined volume and the solid surface immersed in the temperature controlled bath.
Capture bubble technique for contact angle
Capture bubble technique:
- The contact angle is measured between an air bubble of defined volume and the solid surface immersed in the temperature controlled bath.
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.
Capture bubble technique for contact angle
Capture bubble technique:
- The contact angle is measured between an air bubble of defined volume and the solid surface immersed in the temperature controlled bath.
