Questions tagged [diffusion]
For questions referring to the process by which molecules move as a result of their kinetic energy.
66
questions
15
votes
1answer
326 views
Is there a consensus on whether amber can seal in air bubbles for millions of years?
I read about deriving the oxygen content in the atmosphere in the Cretaceous by analyzing the air trapped in bubbles in amber from that period, but I have also found papers disputing the study's ...
12
votes
2answers
452 views
How is the water escaping?
I have a number of toy balls filled with a liquid-presumably water or something else non-toxic. Over about a year, the balls have gone from firm to indented as shown. The water level in one is visibly ...
11
votes
4answers
9k views
Why is entropy favorable?
I cannot seem to grasp the logic behind it. We say that more entropy (or more disordered system) is favorable over less entropy. But why? Why is randomness preferred over proper arrangement of atoms/...
7
votes
3answers
2k views
When is diffusion steady-state?
Fick’s first law of diffusion can be used when $\mathrm{d}C/\mathrm{d}t=0$. If the concentration gradient is time-variant, then we must use Fick’s second law. However, when is diffusion steady-state ...
5
votes
1answer
52 views
How to make equal density pool dye without diffusion
I have a leak I have been struggling to find in my in-ground pool. I am losing about 1/4" of water per day and I believe the issue is one of the fittings in the bottom of the pool. There are two ...
5
votes
0answers
145 views
How to account for the interface between two different phases in a discretized diffusion model?
I have tried to set up a model for the diffusion of a gas into a liquid. The two media are next to each other and the geometry is spherical because the system should simulate the diffusion out of a ...
4
votes
1answer
217 views
Diffusion across a thin film
Given this concentration profile, I am to (1) determine at which side substance A will diffuse out of the membrane and (2) chose one of the following profiles below which represents the concentration ...
4
votes
1answer
85 views
Modelling pH as function of time and position
I already posted this on the Engineering Stack Exchange under Chemical Engineering as I don't know where it's a better fit.
I have a system whose initial state consists of three different sections ...
4
votes
0answers
138 views
Reaction-diffusion equation using Lattice-Boltzmann method
I'm struggling to get a working LBM reaction-diffusion implementation. I have a pretty weak mathematics and physics background, and thus I'm having a hard time understanding the notations around the ...
3
votes
1answer
242 views
Are there any solids that undergo diffusion?
Are there any solids that undergo diffusion?
Introduction:
In school, my teacher is only talking about diffusion happening in liquids and I though that it may be able to happen to solids also.
...
3
votes
5answers
519 views
Where to buy/How to build a Devanathan-Stachurski cell?
I'm preparing to do some hydrogen permeation tests to calculate the diffusivity of some iron samples. I would like to do this using the Devanathan-Stachurski cell, which consists of two cells, one ...
3
votes
2answers
488 views
Is pressure equalization “diffusion”?
There is another question on the site which is:
Two containers, A and B, connected by a closed valve were evacuated and the Chlorine gas (Cl2) with a pressure of 200kPa was put into A and Oxygen (...
3
votes
1answer
136 views
Is there a known relation between the diffusion of a molecule and the diffusion of the reactants?
I have a plasma consisting of elements A and B (which have a similar mass). These elements can react to form molecules AB.
$$
\ce{A + B -> AB}
$$
Now if my plasma or gas is expanding based on ...
3
votes
1answer
138 views
Cause of imaginary part of Warburg element in EIS
In electrochemical impedance spectroscopy experiments (EIS) the diffusion of electroactive species at flat, macroscopic electrodes is commonly described by the Warburg element with the impedance ...
3
votes
1answer
116 views
What kind of unit is this [A*m^2/mol]?
I found a term on this website that gives a unit of [A*m^2/mol]. The equation is:
$k_d^{eff}=\frac{2RT\kappa^{eff}}{F}(t^+-1)\left(1+\frac{\text{d}\ln(f)}{\text{d}\ln(c)}\right)$
where R is the gas ...
3
votes
0answers
26 views
A physical meaning of the Danckwerts boundary condition in terms of concentration profile (adsorption)
Can someone please explain to me in a simplified way, what exactly does a well-recognized Danckwerts boundary condition do to the concentration profile of adsorption phenomena? I just cannot find a ...
3
votes
0answers
25 views
Molecular diffusivity of a three-component mixture
I usually used a following expression by Fuller & Schettler & Giddings to predict molecular diffusivities of binary mixtures:
\begin{equation}
D_i = \dfrac{1\cdot 10^{-4}\left(0.00143\cdot T^{...
3
votes
0answers
32 views
Are there materials permeable to oxygen and carbon dioxide but not nitrogen?
Are there materials permeable to $\ce{O2}$ and $\ce{CO2}$ but not $\ce{N2}$?
Would this even be possible in theory? Can it be done at even at pressures of 300 atm?
3
votes
0answers
44 views
Limit to volume change in a discretized mathematical model?
I have set up a mathematical model describing the diffusion of ozone out of a gas bubble. The bubble is surrounded by a thin gas film. So actually, the model describes the diffusion of ozone through ...
2
votes
2answers
395 views
Determine direction of mass transfer
For part a) I don't really know what to look at. Diffusion occurs from low to high concentrations so by looking at $t_1$ I would say that the direction of the mass transfer is from phase II (organic ...
2
votes
2answers
101 views
how to display diffusion to a young audience?
I need to display some science topic pertaining to the atomic bombs, or atomic history in some way to children of approximately middle school level. So I've decided that something interesting to cover ...
2
votes
2answers
74 views
What is b in the Gompertz model?
I am wondering how to use the Gompertz model in modelling drug diffusion:
$$x(t) = x_\mathrm{max}\exp(-a\exp(b\log t)).$$
$x_\mathrm{max}$ is to be taken as $100,$ $a$ is the undissolved proportion at ...
2
votes
1answer
35 views
Do the concepts of Newtonian and non-Newtonian fluids still hold at a nanoscopic level?
Recently I've spent a great deal of time thinking about enzyme diffusion in cytoplasm. One thing that keeps me awake at night is the constant referral to cytoplasm as a non-Newtonian fluid (which it ...
2
votes
1answer
31 views
How do I compute the mean dissociation time from the dissociation constant, assuming instantaneous binding reaction rate?
There is a solution with two chemical species, A and B. Given the equilibrium dissociation constant
$
K_D=\frac{[A][B]}{[AB]}
$
how do I relate the mean dissociation time $t_{off}$ to $K_D$, and ...
2
votes
1answer
977 views
Why does water diffuse from a lower solute concentration to a higher one?
I'm studying biology and this came up in the context of passive transport. I understand why, in general a gas moves down a concentration gradient (the random movement of gas particles and probability)....
2
votes
0answers
14 views
What unit and dimension for membrane permeability / permeation expression?
Context
When studying the diffusion of gases through thin membrane, one may use the diffusion coefficient $D$ used in Fick's laws as (one dimension form):
\begin{equation}
J = - D \cdot\frac{d\varphi}{...
2
votes
0answers
18 views
Porous materials: Is the expression of a mean free path $\overline{\ell}$ of the gas molecule valid for a binary mixtures?
Suppose that I have a binary gas mixture at low pressure. Can I calculate the mean free path $\overline{\ell}$ of the gas molecule in a pores of solid material by substituting a molecular weight of a ...
2
votes
0answers
36 views
Boundary conditions in a closed system for diffusion (2nd Fick's law) in a porous medium
I would like to calculate the chloride redistribution in concrete (assumption: porous and homogeneous structure, purely diffusion-controlled mass transfer, closed system). For this I have derived a ...
1
vote
1answer
1k views
How can I get self-diffusion coefficient from MSD (mean sqaure displacement)
I run some molecular dynamic simulations for polymer system (liquid), and I have data of MSD vs t(time). MSD-t relation looks linear proportional. Now, which one is the proper way to calculate the ...
1
vote
1answer
401 views
Dependency of rate of diffusion on mean free path and velocity of molecules? [duplicate]
What is the dependency of rate of diffusion ( Graham's law) on mean free path, molecular size , average velocity, collision frequency and pressure? I have searched everywhere on internet but just ...
1
vote
1answer
29 views
Why does ink form tendrils on wet paper [closed]
When a drop of ink or watercolour paint is placed on wet paper, rather than uniformly diffusing in all directions it forms tendrils which rapidly expand out from the spot.
Why is this and what is the ...
1
vote
1answer
41 views
Effective diffusion coefficient of a species in a heterogeneous mixture
I want to calculate the effective diffusion coefficient of hydrogen radical in a mixed-phase consisting of metal oxide, metal hydride and metal. I consider the interfaces to be diffuse.
How can I ...
1
vote
1answer
159 views
Derivation of formula used in calculating molecular mass from a mixture
In a solution to a problem, to calculate Molecular mass of unknown gas from a mixture containing $\ce{O2}$ in $80~\%$, the author used the formula :
$\frac{1}{\sqrt{M_\text{mix}}}=\frac{X_{\ce{O2}}}...
1
vote
1answer
3k views
Graham's law of diffusion
The rates of diffusion of two gases $\ce{A}$ and $\ce{B}$ are in the ratio $1:4$. If the ratio of their masses present in the mixture is $2:3$, the ratio of their mole fraction is?
My tries:
$\frac{...
1
vote
0answers
22 views
Brownian motion of asymmetric particles
The classic Stokes-Einstein relation ($D=k_BT/(6\pi\eta r)$) it is assumed that the particle undergoing Brownian motion is spherical, and experiences ordinary Stokes flow between (or during) ...
1
vote
0answers
26 views
Diffusion coefficient for acid base reaction
I am trying to model an acid-base neutralization reaction. There are two adjacent slabs - one acid and one base. Each slab is the same thickness and I'm only looking at one-dimensional diffusion/...
1
vote
0answers
23 views
Hydrogen permeation from low to high absolute pressure area due to difference in partial pressure?
I need to get help with a question regarding hydrogen permeation which I do not really how to get around.
The statement is as following:
A steel container contains helium gas at high pressure (100 ...
1
vote
0answers
33 views
When is it realistic to model diffusion without advection?
Brownian motion abounds in nature, but so does drift current. In theoretical models of solutions and colloidal suspensions it is sometimes assumed that advection is negligible. I'm wondering if this ...
1
vote
0answers
38 views
Is it easier to mix a solution that is in the liquid or in the vapor phase?
In the article on dilution, Wikipedia has the following statement:
The same direct relationship applies to gases and vapors diluted in air for example. Although, thorough mixing of gases and vapors ...
1
vote
1answer
53 views
Indirectly Measuring Diffusion of NO2 into Water
I am working on a data science project with the central hypothesis is that air-born pollutants can become water-born through diffusion. The example I am using as a proof of concept is $\ce{NO2}$. The ...
1
vote
0answers
20 views
Algae Cultivation Using Polyethylene Film Reactor Vessel Floating In Pool with Algicidal ClO2 Concentration
I'm attempting to grow chlorella algae in a polyethylene film bag (which contains the growth medium) submerged in a temperature-moderating, above-ground swimming pool. This presents a challenge in ...
1
vote
0answers
39 views
Tungsten-Rhenium alloy connections to Copper. Methods?
I want to use some fine WRe wire (50 micron diameter, 80/20) in a particular environment where it has to conduct a few millamps of current while supporting 100g of mass. The problem being the contact ...
1
vote
0answers
45 views
What makes the odor of flatulence fade away? [closed]
If an individual expelled their intestinal gas in an enclosed space, that smell would seemingly fade away. Why is this so? I would think of 3 possible explanations:
The gases dilute in the air. If ...
1
vote
0answers
42 views
Fick's second law atomic diffusion question [closed]
I have a question about diffusion and i'm not sure how to tackle it.
A new processing method is being tried for a lithium battery cathode material -LiCoO2. the process leaves the ceramic oxygen ...
1
vote
0answers
129 views
Molar flux through membrane
Consider a liquid of pure $\ce{A}$ covered by a composite membrane through which
the vapor of $\ce{A}$ is diffusing. The membrane consists of two layers of thickness $l_1$ and $l_2$. The diffusion ...
1
vote
0answers
30 views
Diffusion equation I can't seem to find anywhere
So I was looking through some old lecture notes from a few years back when I stumbled upon a useful and simple equation that should, according to my former professor describe the diffusion in solid ...
1
vote
0answers
75 views
What coefficients can I use in a Reaction-Diffusion Equation?
I am qualitatively analyzing an abstract chemical reaction
$A + B \rightarrow 2A$
where $A$ and $B$ are some liquid substances that undergo Reaction-Diffusion with a second order reaction (actually ...
1
vote
0answers
273 views
How to write mass balance equation for the scenario when molecules are getting generated inside an sphere?
I am working on the physical understanding of diffusion through a nano machine(a shell with hollow interior in which molecules are produced) and have taken few assumptions:
There is rate $r_T$ with ...
0
votes
1answer
728 views
Is the solution isotonic, hypertonic, or hypotonic?
The problem is shown below. I don't understand how can the tonicity of a solution be determined if the solution has multiple solutes dissolved in it.
Do I take the total sum of the mass of solutes ...
0
votes
1answer
190 views
Permeability of containers to poop smells
I do a lot of hiking trips that involve driving for an hour or two to the trailhead in the predawn hours. I'm a morning pooper, so I need to poop during the drive. Before COVID, I would usually plan a ...
