Help understanding the basics of surface tension?

[nitroblu]

We're learning about the basics behind surface tension in chemistry and I'm having trouble applying what I learned. I've gone through my class notes over and over for the past 4 hours and I can't figure out how to even begin to solve the problem!

Can I have some help understanding the basics behind these practice question?

Homework Statement

Suppose an oil film is separated from a liquid film by a loop of flexible string (secured at the edges). Initially, γ_oil = γ_liq = 15 mJ/m² .

a) If we increase the liquid surface tension to γ_liq= 72 mJ/m², what will happen to the oil film?

b) If the area of the oil film increased by 1 cm², calculate the changes in the free energy of the oil film, liquid film and system (oil + liquid), and discuss the practical implications in terms of thermodynamic principles.

Homework Equations

γ = F/L
γ = dG/dA
γ = w_i(z_surface - z_bulk)/2a₀

The Attempt at a Solution

I'm sorry I don't even know where to begin! I'm not really looking for the answers but how I can begin to solve them.

 

[nitroblu]

Can anyone point me at least in the right direction?

Btw, I forgot to upload the picture that goes along with the question in my first post.
http://img705.imageshack.us/img705/1567/unledssv.jpg

Here's what I have so far:

a) Increasing the surface tension of the liquid results in increased inward surface tension force, causing the liquid molecules at the surface to move into the bulk. This means oil molecules in the bulk will need to move to the surface to fill the void, resulting in the oil film stretching out to the secured edges of the string (and thus the cross-section of the oil film decreases in width).

However, I have no clue how to incorporate the values of 15 or 72 mJ/m² into my answer. Also, will γ_oil reach equilibrium to 72 mJ/m²? If so, how? I'm completely lost on these points :(!

b) It's a closed system. So an increase of 1 cm² of the oil film will result in a 1 cm² decrease of the liquid film, right? This is how I've calculated it.

dG_oil = γ_oildA_oil
γ_oil = 15 mJ/m²
dA_oil = 1 cm² = 0.0001 m²
∴dG_oil = 1.5x10^-3mJ

dG_liq = γ_liqdA_liq
γ_liq = 15 mJ/m²
dA_liq = -1 cm² = -0.0001 m²
∴dG_liq = -1.5x10^-3mJ

dG_system = dG_liq + dG_oil = 0

Are my calculations right?
If so, does this mean the practical implications is that the system follows the first law of thermodynamics? I feel there's more of that answer than I can currently think of.