Re: [Phys-l] Stuck powder

[John Denker <jsd@av8n.com>]

On 07/16/2010 09:10 AM, curtis osterhoudt wrote:
> I have a suspension of diamond nano-powder (let's say, though it works for many
> types of powders) in distilled water (deionized or not). This is put on a
> typical glass microscope slide, and allowed to dry under normal conditions.
>
> After drying (I suspect even this definition could get argued about), the powder
> is stuck to the slide, until mechanically dislodged. Once dislodged, the powder
> is no longer "stuck" to the slide, at least for most macroscopic disturbances.
>
>     Why?

Note that diamond dust is an interesting choice, because its particles
typically have lots of flat facets, ideal for sticking to a flat surface
if they get the chance.

>    [1]  Upon "drying", is there a significant amount of water left behind to serve as
> a mono-layer glue?  

One might think that at a late -- but not final -- stage of the drying process,
this would occur.  Surface tension is very strong.  As Feynman remarked, at
the beach the completely dry sand is fluffy, and the completely wet (underwater)
sand is also fluffy, but the partially wet sand is firm, held together by
surface tension.

See also below.

The problem with this theory is that ordinary diamond is famously hydrophobic.
So, it is worth doing a preliminary experiment:  If you put a drop of water in
contact with some powder, does it wet the powder?

Either way I am somewhat confused:
 -- If it wets, then that is unusual for diamond.
 -- It it doesn't wet, then the obvious theory for how water helps the powder
  stick to the substrate has problems.

Also you can check a related part of the theory by repeating the experiment
using a _slide_ that has been coated to make it hydrophobic.
 
>    [2]  Does the water get much of the powder "close enough" -- to the slide, and to
> neighboring grains -- to allow shorter-range forces to come into play?

Assuming a wettable powder, then as mentioned in item [1], there is a stage
where a tiny amount of water is holding a powder facet against the substrate
surface.  As this water finally goes away, this is the perfect set-up for
the formation of direct powder-substrate bonds.

>    [3]  Is there enough of the glass (or powder) material dissolved to serve as a
> glue matrix?

Quite possibly, but that is not the first thing that happens.

Note that ordinary glass is somewhat soluble in water.   Even pure quartz is 
soluble.  This is not particularly noticeable on small timescales and large 
lengthscales, but remember that _agate_ is silica that has been deposited
out of aqueous solution.

Also note that glass slides out of the box will have undergone a certain amount
of surface reconstruction and passivation.

So ... (probably) on a very short timescale the water etches away a monolayer
and thereby activates the previously passivated surface.  This makes it much
easier for strong substrate-powder bonds to form.

Also on a short timescale the water dissolves any thin layer of impurity that
might be present.

Then ... (maybe) on a longer timescale the water dissolves enough glass to make
proper glue.  Sodium silicate makes an excellent glue.

You can check the "glue" hypothesis by doing xray elemental analysis on the
formerly-stuck powder scraped off the slide.  If you find lots of silicon in
your diamond powder, you know where it came from.

>   [4]  Is there enough air absorbed to serve as a glue matrix?

I don't see how.  Air is not glue, especially when surrounded by more air.
Think of the fluffy underwater sand.

==============

It is well known that two clean, optically-flat pieces of metal will "cold
weld" if placed in contact.  For large flats, it make take some doing to
wring out the last of the air that separates them, but if you do get them
into contact they will never come apart again.

In the case of metals I would be tempted to monitor the process by watching
the capacitance and the DC conductivity.

===

Note that this is tangentially related to static electricity.  Think about the
electric field : delta(work function) divided by small distance.

Note that this shows yet again that there is no meaningful dividing line between
physics and chemistry.  At medium-small distances we have a humongous electrical
force, and as the distance becomes even smaller it becomes indistinguishable from
a chemical bond.

===

You can almost certainly do without the water, if you use a "sand blaster" setup
to blast the powder against the surface.  The idea is to impart enough momentum
to the powder particles that they actually contact the surface, cutting through
any air cushion or other layers.  If/when they truly make contact, they will
stick.  Think of the dust that sticks to fan blades.

Returning to the previous tangent:  Blowing dust is a tremendous way of making
static electricity.  But that's a topic for a separate discussion.