Tutorial on static electricity in grinders

Grinders are one of the keys to exceptional espresso. Discuss them here.
toobs1234
Posts: 23
Joined: Thu Jan 24, 2013 8:52 pm
Real Name: John
Equipment: Slayer/K30 Vario
Location: Zurich

Postby toobs1234 » Sun Feb 03, 2013 11:22 am

I hope someone finds this interesting/useful. It's about half copy/pasted from Wikipedia and half original:

Contact induced charge separation

Materials are made of atoms that are normally electrically neutral because they contain equal numbers of positive charges (protons in their nuclei) and negative charges (electrons in "shells" surrounding the nucleus). Electrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. Upon separation, they retain this charge imbalance. This is known as the triboelectric effect and results in one material becoming positively charged and the other negatively charged. The triboelectric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together (e.g., fur against an acrylic rod). Contact-induced charge separation causes your hair to stand up and causes "static cling" (for example, a balloon rubbed against the hair becomes negatively charged; when near a wall, the charged balloon is attracted to positively charged particles in the wall, and can "cling" to it, appearing to be suspended against gravity).

Triboelectric series

The polarity and strength of the charge on a material once they are separated depends on their relative positions in the triboelectric series. A material towards the bottom of the series, when touched to a material near the top of the series, will attain a more negative charge, and vice versa. The further away two materials are from each other on the series, the greater the charge transferred. Materials near to each other on the series may not exchange any charge, or may exchange the opposite of what is implied by the list. This depends more on the presence of rubbing, the presence of contaminants or oxides, or upon properties other than on the type of material. Lists vary somewhat as to the exact order of some materials, since the charge also varies for nearby materials.

Positive(+)
Human Hands
Asbestos
Rabbit's Fur
Glass
Human Hair
Mica
Nylon
Wool
Lead
Cat's Fur
Silk
Aluminum
Paper
Cotton
Steel
Wood
Lucite
Sealing wax
Amber
Polystyrene
Polyethylene
Rubber balloon
Sulphur
Hard rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Sulfur
Acetate, Rayon
Polyester
Celluloid
Polyurethane
Polyethylene
Polypropylene
Vinyl
Silicon
Teflon
Saran Wrap
Negative (-)

Charge backflow

During separation, some of the charge flows back reducing the amount of static electricity. The amount of backflow depends on the degree to which both materials are conductors. If both materials are strong conductors, the amount of backflow will be great and very little static electricity will be generated. Conversely, for significant static electricity to be generated, at least one of the materials must have a high resistance to the flow of electrons (be an insulator). The amount of charge backflow also depends on the speed of separation: slower speed equals more backflow.

Static electricity from coffee grinding

In coffee grinding, we have two materials, coffee beans and steel. Unfortunately I can't find a triboelectric series with coffee beans in the list. For the purpose of this discussion, I'll assume that coffee beans lie further up the list (more positive) than the steel of the burrs. In this case the grinds will take on a positive charge (i.e. they will be stripped of electrons) and the burrs will take on a negative charge. If the burrs are grounded, the extra electrons will flow to ground and the burrs will remain nuetral, but the positive charge will remain on the grinds. As noted above, at least one of the materials must be an insulator for static electricity to be generated. This is why humidity matters. The conductivity of coffee grinds increases as the water content of the beans increases. Spraying water on the beans helps increase conductivity and therefore reduces static. The speed of separation also affects the amount of charge backflow. Slower grinding results in less static.

Induced charge (static cling)

When a charged object is brought near an uncharged, electrically conducting object, such as a piece of metal, the force of the nearby charge causes a separation of positively charges nuclei and negatively charged electrons. For example, if a positive charge is brought near a metal object, the electrons in the metal will be attracted toward it and move to the side of the object facing it. When the electrons move out of an area, they leave an unbalanced positive charge due to the nuclei. This results in a region of negative charge on the object nearest to the external charge, and a region of positive charge on the part away from it. These are called induced charges. If the external charge is negative, the polarity of the charged regions will be reversed. The overall charge of the metal object has not changed, but because the attraction between positive and negative charges falls off quickly with distance, the force of attraction between the charged particle and the near region of the metal object is stronger than the repellant force between the charged particle and the far part of the metal object. The net result is that the charged particle is attracted to the uncharged metal object.

A similar induction effect occurs in nonconductive (dielectric) objects, and is responsible for the attraction of small light nonconductive objects, like scraps of paper or Styrofoam, to static electric charges. In nonconductors, the electrons are bound to atoms or molecules and are not free to move about the object as in conductors; however they can move a little within the molecules.

If a positive charge is brought near a nonconductive object, the electrons in each molecule are attracted toward it, and move to the side of the molecule facing the charge, while the positive nuclei are repelled and move slightly to the opposite side of the molecule. Since the negative charges are now closer to the external charge than the positive charges, their attraction is greater than the repulsion of the positive charges, resulting in a small net attraction of the molecule toward the charge. This is called polarization, and the polarized molecules are called dipoles. This effect is microscopic, but since there are so many molecules, it adds up to enough force to move a light object like a coffee grind.

Managing static electricity in a coffee grinder

In order to reduce static electricity in the coffee grinds, we need to replace the electrons that the have been stripped from the grinds. This can be done in two ways. First, by grounding any metal surfaces that the grinds come in contact with (in the grinder and in the portafilter). Second, by implementing an electrically conducting path between the burrs, which have absorbed the electrons that have been stripped from the grinds, and the metal surfaces that the grinds come in contact with. Painting or otherwise insulating the metal surfaces that the grinds come in contact with will remove any possibility of neutralizing the static charge that built up during grinding.

Now, if you have followed what I've written and are a critical thinker, you should be saying: "but wait a second, isn't encouraging more contact with metal and subsequent separation going to generate more static rather than less?" The answer to that is yes and no. Yes, there will be more separation induced charge, but because the speed of separation is much slower than in the burrs and because the grinds are already positively charged and the metal is negatively charged (assuming a conductive path to the burrs), the charge backflow will dominate the charge induction and static on net will be reduced.

User avatar
RapidCoffee
Team HB
Posts: 3190
Joined: Sun Dec 11, 2005 8:05 pm
Real Name: John Weiss
Equipment: Mazzer Robur, LaSpaziale S1, plus every coffee brewing device invented to date ;-)
Location: Rapid City, SD

Postby RapidCoffee » Sun Feb 03, 2013 12:58 pm

toobs1234 wrote:In coffee grinding, we have two materials, coffee beans and steel. Unfortunately I can't find a triboelectric series with coffee beans in the list. For the purpose of this discussion, I'll assume that coffee beans lie further up the list (more positive) than the steel of the burrs.
...
Now, if you have followed what I've written and are a critical thinker...


As a critical thinker who has followed what you are saying :lol:, I cannot help but note that coffee beans are plant products, and most closely resemble materials near the center of this list: wood, cotton, and paper. Steel and aluminum also lie near the center of the list. According to Wikipedia, they have no charge (steel), a small positive charge (paper), or a small negative charge (wood). Yes, it is possible that coffee beans have a strong positive triboelectric charge. But there is nothing on the Wikipedia list that would suggest this. The burden of proof lies on you...

For completeness, here are the (extensively quoted) Wikipedia links:
http://en.wikipedia.org/wiki/Static_electricity
http://en.wikipedia.org/wiki/Triboelectric_effect
http://en.wikipedia.org/wiki/Electrostatic_induction
John

toobs1234
Posts: 23
Joined: Thu Jan 24, 2013 8:52 pm
Real Name: John
Equipment: Slayer/K30 Vario
Location: Zurich

Postby toobs1234 » Sun Feb 03, 2013 1:13 pm

Yes, it is possible that coffee beans have a strong positive triboelectric charge. But there is nothing on the Wikipedia list that would suggest this. The burden of proof lies on you...


Sorry, I'm really confused by your challenge. Could you clarify by any chance? I said in my post I don't know where coffee beans lie on the series relative to the burrs. For the purpose of exposition, I assumed that they are more positive than the burrs. It doesn't matter, though, if they are more positive or negative. If they in fact are more negative, then everything I wrote still holds, just in reverse: the grinds will be negatively charged and the burrs will be positively. Everything else is exactly the same.

User avatar
RapidCoffee
Team HB
Posts: 3190
Joined: Sun Dec 11, 2005 8:05 pm
Real Name: John Weiss
Equipment: Mazzer Robur, LaSpaziale S1, plus every coffee brewing device invented to date ;-)
Location: Rapid City, SD

Postby RapidCoffee » Sun Feb 03, 2013 1:42 pm

Your argument is a classic example of circular reasoning.

I have no problem with the following:
observation: the HG-1 grinder generates large amounts of static
hypothesis: this is caused by the large difference in triboelectric charge between steel and coffee beans
You could then devise experiments to test your hypothesis.

Instead, you seem to be saying:
assumption: there is a large difference in triboelectric charge between steel and coffee beans
hypothesis: this is causing the large amounts of static seen in the HG-1 grinder
Your assumption needs to be tested, before you can make any conclusions about your hypothesis.

Incidentally, it's easy to come up with alternative hypotheses for the static. Surprisingly, human hands lie at the very top (most positively charged materials) of the triboelectric list. The prolonged contact with human hands in a hand grinder might be responsible for the buildup of static charge. In support of this, I note that static charge is not a significant problem for my Robur or, apparently, other motorized grinders using this burrset.
John

coffeedom
Posts: 158
Joined: Fri Jan 15, 2010 7:24 pm
Real Name: Dom
Equipment: too much
Location: Canada

Postby coffeedom » Sun Feb 03, 2013 2:05 pm

Thank you John (toobs) for the concisely written exploration of static. How it pertains to grinding in general and the HG One specifically needs further exploration, in that the differnce in triboelectric charge between the grinds and the burrs needs to be established as significant. It probably is though, so what you are saying could be right. The anodized aluminum could also play a role.

I for one appreciate that you took the time to summarize where you are coming from.

As for the effect of hands, maybe TomC can jump in here as he's used the grinder hands-free and can comment on whether that made any difference to the static or lack thereof.

User avatar
TomC
Team HB
Posts: 4605
Joined: Mon Jun 06, 2011 1:46 pm
Real Name: Tom Chips
Location: San Francisco

Postby TomC » Sun Feb 03, 2013 2:46 pm

No, the drill created a ton of static too.
California Wine in 70's, Craft beer circa 00's, Specialty Coffee......

toobs1234
Posts: 23
Joined: Thu Jan 24, 2013 8:52 pm
Real Name: John
Equipment: Slayer/K30 Vario
Location: Zurich

Postby toobs1234 » Sun Feb 03, 2013 2:48 pm

RapidCoffee wrote:The prolonged contact with human hands in a hand grinder might be responsible for the buildup of static charge. In support of this, I note that static charge is not a significant problem for my Robur or, apparently, other motorized grinders using this burrset.


Ahhh, that's what you meant! I understand now. :) One thing you might want to check regarding this theory is whether or not there is a conductive path between the handles of the HG-1 and the grinds.

I take your point about there being multiple possibilities for the source of the static and your skepticism is warranted. For me personally, it's self-evident that the millions of high-speed, high-pressure contact/separation cycles taking place between the burrs are the source of the static, but that's my opinion, rather than an absolute truth as you correctly pointed out. Static is generated by all grinders in dry conditions--hand ground or electric. The different end results from different grinders is the management of the static after the grinds leave the burrs. (the caveat here, of course, is that speed matters).

One thing I want to make clear, though, is I'm not postulating that there is a large difference in charge affinity between the metal burrs and the coffee grinds. You just need some difference. The huge amount of contact/separation cycles going on within the burrs is what makes the difference in this case.

thanks for keeping me honest, John

toobs1234
Posts: 23
Joined: Thu Jan 24, 2013 8:52 pm
Real Name: John
Equipment: Slayer/K30 Vario
Location: Zurich

Postby toobs1234 » Sun Feb 03, 2013 3:05 pm

TomC wrote:No, the drill created a ton of static too.


That seems to rule out hands as being the source of static. Are there other potential sources of the static?

User avatar
Terranova
Posts: 436
Joined: Thu Mar 15, 2012 9:24 pm
Real Name: Frank
Equipment: Slayer v.2, Synesso, D&L, EK 43
Location: Germany Cologne / West Indies

Postby Terranova » Sun Feb 03, 2013 3:54 pm

toobs1234 wrote:For me personally, it's self-evident that the millions of high-speed, high-pressure contact/separation cycles taking place between the burrs are the source of the static



+1


Lets try to find a way to neutralise these little animals, destroying our mood on a monday morning. :)

Will it make a difference to fit a lower funnel in steel, connected to a metal wire which is grounded to the sink or so ?
Or do you habe another practical suggestion which is worth it, trying out.
In 2 weeks time or so I get my hands on a HG One, which I cannot take apart (it's not mine) , but just changing the funnel is quiet ok.
It has also been mentioned, some beans are prone to produce more static, but this is more because of coffee being very hygroscopic, translated: it exchanges watermolecules, gives them up to the atmosphere or absorbs them, depending on relative humidity surrounding the grinder.
It is worth a try.

Cheers

Frank

SeanSheep
Posts: 3
Joined: Mon Jan 14, 2013 3:02 am
Real Name: Sean
Equipment: Rocket Giotto, Mazzer SJ-e
Location: Sydney, Australia

Postby SeanSheep » Sun Feb 03, 2013 5:12 pm

Thanks John for the post, I found it really interesting. I for one am glad you're throwing your ideas out there.

Just as an experiment I tested the grind path on my Super Jolly-e. I found that everything that the grinds come into contact with has a conductivity path back to the burrs. The funnel, the portafilter holder (and by extension the portafilter and basket), even the grind catching tray, all had continuity to the burrset.

Thanks,
Sean.