Can a small amount of smoke be dense enough to stay in the air keeping its shape for a minute or so?
Or does it always dissipate quickly?
I read this and think to myself "optimization problem". Firstly, you should know the following, which is the law of diffusion:
$$\frac{\partial \phi}{\partial t} = D\,\frac{\partial^2 \phi}{\partial x^2}$$
For clarity, $\phi$ is a function that represents the distribution of the concentration of the gas. It is a scalar function of 3 variables, which is to say that I could write it as $\phi(\vec{r})$, where $\vec{r}$ represents typical $x,y,z$ coordinates. If have an image in your mind of a cloud that is the shape of a snowman, that can be represented by that function, so can smoke rings or whatever you desire.
The clarity of the shape degrades over time, exactly per the above diffusion equation. Picture blurring an image in Photoshop. That is very similar to the process that happens.
- Q: Can you reduce the rate at which this blurring happens?
- A: Yes you can
The rate at which $\phi$ (your snowman) degrades in sharpness comes from the magnitude of $|d\phi/dt|$. This magnitude is proportional to the diffusion coefficient $D$ as well as that other derivative with respect to $dx^2$, but that term is representative of the sharpness itself, so we don't want to reduce that, we would rather reduce $D$. In order to reduce $D$, we need to first talk about mean free path and velocity of the gas molecules. I'll use this source and refer to the mean free path $\lambda$ (units of length) and average speed $\bar{c}$. In general D is proportional to those two.
$$D \propto \lambda \bar{c}$$
For a gas cloud the parameter $\lambda$ has mostly to do with the density of the gas, as well as some other things. Again, we would like to minimize $D$, but $\lambda$ might not have much design freedom. On the other hand, $\bar{c}$ could have great design freedom. This is also dependent on the temperature of the gas, but more specifically, the temperature is a measure of the kinetic energy of the molecules. I'll say fairly generally:
$$\frac{1}{2} m \bar{c}^2 = \frac{3}{2} k T$$
Never mind very much what $k$ is (it's just a physical constant), what matters is that this equation has temperature $T$ and $m$. I'm taking your question to be most likely concerned with normal air. That means that it is unlikely that we would have $T$ as a design variable. However, since you are not specifying the gas you are working with, it's possible we could choose that, and the selection of that gas determines $m$ which is a factor in determining $\bar{c}$ which is a factor in determining $D$, which determines the persistence of your cloud image.
Bottom line: Heavier gases will diffuse more slowly, meaning the image will persist longer.
An example of a high molecular weight gas is common refrigerant gases, like R-134a. If you released that into the air it will diffuse rather slowly compared to other examples. NOTE: don't do this, it would be dangerous and probably illegal.
There are 3 kinds of mixtures in liquid...
- True Solution
- Colloids
- Suspension
These three vary in between because of the size of the particle in them. see wiki. Now, the salt solution you were talking about comes under category "true solution" i.e. particle size less than 1 $nm$.
Now we don't have sieve to filter out this particles of this dimension. Even bacterias and microbes are orders of magnitude greater than this dimension.
As per classical textbooks, it's only suspension that can be filtered out using sieve or sedimentation. Even for colloids one needs ultrafiltration methods, with superfine pores in the filter.
Best Answer
You are right, these masks are almost useless as a protection against urban aerosols. With swine flu, there was a lot of discussion (example) that even the best masks cannot catch virus particles which are only 100 nm in size. The usual surgical masks are even less effective - they will hardly block anything smaller that 1 micron.
Now, urban aerosols have several size modes: most numerous are just 10-50 nm in size, although most of the mass will be in large 1-10 micron particles (this is the soot/dust that you can see).
The largest particles are blocked by the mask - but they are also filtered by your nose. The smallest particles - below 200 nm - that are considered much more dangerous because they can reach your lungs and even enter the bloodstream. The mask will also not help against nitrogen dioxide - the major component of urban pollution.