“A Pulsating Heat Pipe (PHP) is a two-phase thermal link that can be used for cooling of hot spots of any kind (microelectronics, car engines, etc.). Its heat transfer capability is known to be extremely high. The PHP structure is simple: it is a capillary tube bent in several branches that meander between a hot part (called evaporator) and a cold part (called condenser), and filled up with a pure two-phase fluid. When the temperature difference between evaporator and condenser exceeds a certain threshold, gas bubbles and liquid plugs begin to oscillate spontaneously back and forth inside the tube and PHP starts transferring the heat.
When liquid/vapor interfaces oscillate, they deposit liquid films inside the tube. The dynamics of such films is known to be a key parameter that rules out the functioning of PHPs. Their behavior has however never been observed directly or explained: the purpose of our work is thus to study these liquid films – so called Landau-Levich films – when a heat flux is applied to them in the evaporator part of a PHP.
Our experimental setup features the simplest, single branch PHP. A liquid/vapor interface oscillates in a straight tube and deposits a liquid film at each passage. The setup allows the film visualization. An original combination of optical techniques lets us measure its 3D profile at all times. Under heating conditions, the film recedes. A dewetting ridge is formed, near the triple contact line. Such a behavior is typical for the isothermal non-wetting films but is surprising in our case: indeed, the liquid we chose wets perfectly the substrate at equilibrium. An explanation of this behavior will be given.”