Simple Substituting and Rearranging

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Discussion Overview

The discussion revolves around the process of substituting and rearranging equations in the context of approximating solutions related to diffusion, particularly focusing on the transition from one equation to another and the implications of certain approximations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • One participant requests clarification on how to derive results from specific equations, particularly regarding an approximation involving delta.
  • Another participant suggests that the image provided may be difficult to read, prompting a request for a larger version.
  • A participant presents an approximation of an equation, stating that it can be simplified under certain conditions, specifically when b=0.
  • Further elaboration is provided on the derivation of the approximation, detailing the assumptions made about the smallness of delta compared to q_0 and the resulting simplifications.
  • Another participant inquires about the context of the problem, speculating that it may relate to Laplace Transforms and the diffusion equation.
  • A later reply confirms that the problem is indeed related to diffusion, referencing Fick's First Law and a specific approximation method used in the context of medicament release from ointment bases.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and progress on the problem, with some providing insights into the derivation while others seek clarification. No consensus is reached on the overall approach or the implications of the approximations.

Contextual Notes

The discussion includes assumptions about the smallness of certain terms and the context of the equations being derived, which may not be fully resolved or defined within the thread.

Caccioppoli
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25qgphi.png


30c86t1.png


Hello,

may someone be so kind to explain how to arrive, step by step, from equation 23 to 28?

Most of all I would like to understand the approximation with delta: if I substitute eq26 in 25 I get a different result (e.g. delta^3 terms).

See the attached image.

Thank you very much.

PS
eq24 may be taken as it is, I mean, phi is simply "(A D Cs / x') - (c D Cs/2)"
 
Last edited by a moderator:
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You may find that people are going to have a hard time reading the image...can you enlarge it?
 
MarkFL said:
You may find that people are going to have a hard time reading the image...can you enlarge it?

Sorry, I've uploaded a bigger version of the image, split in two figures.
 
I can update the problem since I've done some progress.

The following equation

q=aq^3+b [eq#1]

can be approximated with q=a^{-0.5} + \delta [eq#2]

with \delta=-b/2

Where does this approximation come from and why is \delta=-b/2?

Thank you very much.
 
Last edited:
Managed to reach the solution :D

eq.#1 is aq^3+b=q

eq.#1 would be simpler if b=0, the zero-order approximation is

q_0=aq_0^3 so q_0=a^{-0.5}

The next order (1st order) approximation is

q=q_0 + \delta

It is assumed that \delta is small in comparison to q_0 so that all the terms in \delta^2 and \delta^3 are discarded and

q^3=(q_0+\delta)^3≈q_0^3+3q_0^2\delta

So that eq.#1 becomes

q_0+\delta=a[q_0^3+3q_0^2\delta]+b

Recalling the zero-order approximation we have that

\delta=a[3\delta q_0^2]+b

then

\delta=a3\delta a^{-1}+b=3\delta+b

The solution is \delta=-b/2
 
I'm just curious about the context of this problem. It looks like you're doing Laplace Transforms on a PDE (the diffusion equation?). Is that correct?
 
The problem is actually of diffusion.

It starts with Fick's First Law of Diffusion, an ODE (which is steady), after it uses a PSEUDO-Steady State Approximation (small t) to get an approximated expression for fluxes.

To understand more about this kind of approximation I should read the main article which is

"Rate of release of medicaments from ointment bases containing drugs in suspension - Higuchi - 1961"What I've studied till now is a generalization of an expression derived in Higuchi's article of 1961, so I don't know if he starts from a PDE, but probably he does.
 

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