  Boiling crisis is the rapid formation of the quasi-continuous vapor film
between the heater and the liquid when the heat supply exceeds a critical
value. We propose a mechanism for the boiling crisis that is based on the
spreading of the dry spot under a vapor bubble. The spreading is initiated by
the vapor recoil force, a force coming from the liquid evaporation into the
bubble. Since the evaporation intensity increases sharply near the triple
contact line, the influence of the vapor recoil can be described as a change of
the apparent contact angle. Therefore, for the most usual case of complete
wetting of the heating surface by the liquid, the boiling crisis can be
understood as a drying transition from complete to partial wetting. The state
of nucleate boiling, which is boiling in its usual sense, is characterized by a
very large rate of heat transfer from the heating surface to the bulk because
the superheated liquid is carried away from the heating surface by the
departing vapor bubbles. If the heating power is increased, the temperature of
the heating surface increases with the heat flux. When the heat flux from the
heater reaches a threshold value q CHF (the critical heat flux, CHF), the vapor
bubbles suddenly form a film which covers the heating surface and insulates the
latter from the bulk of the liquid. The temperature of the heating surface
grows so rapidly that the heater can fuse unless its power is controlled. This
phenomenon is known under the names of "boiling crisis," "burnout," or
"Departure from Nucleate Boiling" (DNB) [1]. The final state of this transition
is called film boiling. This problem has become very important since the
1940's, with the beginning of the industrial exploitation of heat exchangers
with large heat fluxes (as with nuclear power stations). Since then a huge
amount of research has been done for the various conditions of pool boiling
(boiling without imposed external flow) and flow boiling (boiling of the
flowing water) [2]. Numerous empirical correlations have been proposed, each
describing the dependence of the CHF on the physical parameters of the liquid
and of the heater more or less correctly for a particular geometry and
particular conditions of boiling [2]. A strong dependence of the threshold on
the details of the experimental setup coupled with difficulties in separating
the consequences of DNB from its causes is at the origin of a large number of
frequently controversial hypotheses [2]. The violence of boiling makes
observations quite difficult. Good quality photographic experiments are
presented in only a few articles (see e.g. [3] -- [6]). Despite an increasing
interest in the physical aspect of the problem during recent years [7,8] and
numerous empirical approaches, the underlying physics still remains obscure. In
this Letter, we propose a model based on a non-equilibrium drying transition.
