# What is a bulk phase transition?

I have been able to google "bulk phase transition" and get plenty of results that verify that something called a bulk phase transition exists, however, I cannot seem to find a precise definition of what exactly a bulk phase transition is. Could someone please help me with a definition of bulk phase transition? I would very much appreciate it.

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I'm not an expert so I won't submit it as a definitive answer but when it comes to complex physical terms I find it helpful to break them down into their constituent parts. A phase transition occurs when a material changes phase (liquid -> gas, gas -> solid, etc.). When talking about materials the term "Bulk" generally refers to a volume. So putting the terms together, it would seem that a "bulk phase transition" would be when an entire material changes phase at once. –  Michael Leonard Oct 22 '12 at 20:28
@MichaelLeonard Thanks for the input. I was under the impression when we said "phase transition" we always meant the entire system under consideration was undergoing a change. Do you think "bulk" is simply extra? or is my understanding of normal phase transitions off? –  kηives Oct 23 '12 at 22:18
I have the same confusion as you on that. I did stumble across one paper that mentioned a comparison of Surface Phase transition vs. Bulk Phase transition. That would seem to indicate that there is at least some meaningful difference. If I were to take a guess, I would think bulk phase transition would be when the entire material undergoes a phase transition (i.e. sublimation) vs surface phase transition where just the surface of the material is undergoing a phase transition (i.e. water freezing on a lake) –  Michael Leonard Oct 24 '12 at 3:06
@kηives: Yes, this is by opposition to, e.g., surface phase transitions. A typical example of the latter is the wetting transition. –  Yvan Velenik Oct 24 '12 at 7:05
@MichaelLeonard If you would like to write up an answer I'll approve FYI, since that seems to be the consensus, however small. –  kηives Oct 24 '12 at 14:25

I think the word bulk transition is used in several (related) contexts. The first is the following: Consider a system with a first order phase transition, governed by some partition function in $d$ dimensions. Impose boundary conditions that create a $d-1$ dimensional interface between the two phases. Construct an effective partition function (in $d-1$ dimensions) for the interface. You can now ask whether the roughening transition for the $d-1$ dimensional interface occurs at the same temperature as the $d$ dimensional bulk transition.
A (related) usage of the term bulk transition is in lattice field theory, usually in the context of first order transitions of lattice gauge theories at strong coupling. A weak coupling transition of a $d+1$ dimensional lattice model is related to a thermal phase transition of a continuum field theory in $d$ spatial dimensions. But this is not the case for a strong coupling transition -- there is no continuum limit, and therefore no thermal interpretation of the boundary conditions. The transition is merely a bulk transitions of the $d+1$ dimensional lattice model.