@Preben Alsholm 

Sorry about that. I thought I did.Test11.mw

@Carl Love 

Thank you so much

@dharr 

Thank you for your explanation. I'm okay with a solution without f1 and f2. Thank you all for your contributions. 

@mmcdara 

K[3]+K[1]*cos(theta(x, z, t))^2-K[3]*cos(theta(x, z, t))^2 = f[1](theta(x, z, t)), K[3]*cos(theta(x, z, t))^2-K[1]*cos(theta(x, z, t))^2+K[1] = f[2](theta(x, z, t))

I just replaced them back to avoid non-dimenisonalising the f1 and f2. After non-dimenionsionalising, I can then get them back. 

@mmcdara 

hank you. I'm rather looking for Ericksen number, which is there already. 

Er = |xi|*h^3/K[1], where d is the characteristics length, |xi| (activity coefficient) =P (pressure scale) and K[1] is the elastic constant. 

@dharr 

I have tried adding the params but is still giving same error. Perhaps,  I will try he suggested.

@acer 

Okay, thank you 

@dharr 

I tried to run this on my maple version, but I received:

Error, (in dchange/funcs) unable to change variables in Eval structures where the evaluation equation is not linear in the old variables {K[3], gamma[1], gamma[2]}

I'm using maple 18

@acer 

Thank you so much for your help. I'm okay now.

@acer 

Thank you

@acer 

Thank you. I have sorted this out already.

@mmcdara 

Please, just ignore my last comment. I understand it now.

@mmcdara 

Thank you for this. I wanted something like after I must have collected the coefficient for both 

(alpha[1]*cos(theta(x, y, t))^2*sin(theta(x, y, t))^2+(alpha[2]+alpha[3]+alpha[6]-alpha[5])*((1/4)*cos(theta(x, y, t))^2-(1/4)*sin(theta(x, y, t))^2)+(1/2)*alpha[4]+(alpha[5]+alpha[6])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2)+(alpha[3]-alpha[2])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2))*dudy^2   + (alpha[1]*cos(theta(x, y, t))^2*sin(theta(x, y, t))^2+(alpha[2]+alpha[3]+alpha[6]-alpha[5])*(-(1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2)+(1/2)*alpha[4]+(alpha[5]+alpha[6])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2)+(alpha[3]-alpha[2])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2))**dvdx^2 

But it doesn't collect the coefficients of *dvdx^2.

@mmcdara 

Thank you for this. I don't want them separate. I want to collect the coefficient of dudy^2 and dvdx^2 in the bigger equation. I have been able to collect them separately but this not what I want. 

Please ignore this. 

I tried "add" instead of sum and it worked.