A myth about ice skating

[kowalskil@alpha.montclair.edu]

Do you recall that the tripple point curve for water has a negative 
slope separating the solid and liquid phase? And you know that many 
textbooks tell us that the negative slope (lowering of the melting 
point by pressuse below the blades of a skater) is responsible for 
the formation of liquid water there. According to a new book by Clifford 
Swartz (see below) this is a myth; the pressure is usually too low for 
this. What do you think?
........................................................................
HERE IS WHAT CLIFF SWARTZ SENT ME ABOUT ICE SKATING AFTER THE LAST
REGIONAL AAPT MEETING 

>  From:   APOLLO::WINS%"CSWARTZ@ccmail.sunysb.edu"   30-OCT-1996
>  To:     KOWALSKI@alpha.montclair.edu

> Hello, Ludwik 
>        Let me expand the information about ice skating by citing the
> section on the topic from my new book "Teaching Introductory Physics",
> published by the American Institute of Physics.(p 93)

> "Ice skates experience low friction because a thin water film is
> produced between blade and ice.  A popular legend has it that this
> effect is caused by the lowering of the melting point due to the pressure
> exerted by the blade.  It is true that one of the unusual features of
> water is that it expands during freezing.  Consequently, if we prevent
> the expansion by increasing the pressure, we prevent the freezing and
> thus lowwer the melting point.  The usual pressure-temperature diagram
> for water, Fig 4-5, shows the boundary line between solid and liquid
> arching back to the left from the triple point.  However, the actual
> effect is very small.

> The slope of the boundary line is -(1.2x10^7 N/m^2/C) It would take an
> increase of 120 atmospheres to lower the melting temperature 1 Celsius
> degree. For the typical skate blade, the area is 27 cm x 4 mm = 11 cm
> If the full weight of a skater with mass 65 kg is exerted on one blade,
> the increased pressure would be about 6 atm.  Sharpening the blades
> does not decrease the contact area appreciably, since the blade sinks
> down into the relatively soft ice.

> What does produce the water film between blade and ice? There are two 
> plausible explanations. When the leading edge of a blade strikes the 
> ice, the resulting friction energy can melt a trail for the rest of the 
> blade.  A more important effect stems from a phenomenon first noticed 
> by Faraday and then largely ignored because it was not understood.  
> We now know that at the interface between ice and air there is a fhin 
> film of water. The thickness increases from monomolecular to several 
> hundred molecules as the temperature rises from -10 to 0 C. Since the 
> reduction of friction depends on the the water film, you might conclude 
> that the fastest skating could be done on ice close to the melting point.  
> However, warm ice is soft ice, allowing the blades to sink in more. On 
> the other hand, cold ice, which is hard, has only a thin film of surface 
> water.  These two competing effects yield a minimum of friction for 
> speed skating at about -7 C.  (For further details. see James White, The 
> Physics Teacher, 30, 495 ((1992))."

> Also note that any explanation must account for the very low friction
> experienced by the puck. I hope this explanation will either satisfy
> people or stir up further controversy!