IMECE2005-80920 SIMULATIONS OF RUPTURE IN THIN FILMS OF EVAPORATING LIQUIDS

ABSTRACT Evolution of dry patches in a thin film of a volatile liquid on a uniformly heated plate is investigated in the framework of a lubrication-type model. The effects of surface tension, evaporation, thermocapillarity, and disjoining pressure are taken into account. Dry areas on the plate are modeled by isothermal microscopic films, which are in thermodynamic equilibrium with the vapor. For non-polar liquids such equilibrium is achieved due to van der Waals forces. Simulations indicate formation of a well-defined capillary ridge around a growing dry patch and show increase in the contact line speed with time. For polar liquids the microscopic film is formed by combined action of van der Waals and electrical double layer forces, the capillary ridge is very small and the contact line speed quickly approaches a constant value. Numerical simulations demonstrate that the proposed model is capable of describing a number of complicated phenomena observed in dewetting of evaporating films. INTRODUCTION Moving contact lines in liquid-air systems have been studied extensively. However, a related problem of motion of a contact line between solid, liquid and vapor phase of the same liquid received much less attention. This problem is important for many applications such as micro heat pipes, microfluidic actuators, and spray cooling. Of particular interest for the present study is contact line motion during formation and growth of dry patches in thin evaporating films. Theoretical investigations of dewetting in thin evaporating film

The relation of steady evaporating drops fed by an influx and freely evaporating drops

We discuss a thin film evolution equation for a wetting evaporating liquid on a smooth solid substrate. The model is valid for slowly evaporating small sessile droplets when thermal effects are insignificant, while wettability and capillarity play a major role. The model is first employed to study steady evaporating drops that are fed locally through the substrate. An asymptotic analysis focuses on the precursor film and the transition region towards the bulk drop and a numerical continuation of steady drops determines their fully non-linear profiles. Following this, we study the time evolution of freely evaporating drops without influx for several initial drop shapes. As a result we find that drops initially spread if their initial contact angle is larger than the apparent contact angle of large steady evaporating drops with influx. Otherwise they recede right from the beginning

PREFACE: DROPS, BUBBLES, AND THIN FILMS, SPECIAL ISSUE HONORING PROFESSOR OLEG KABOV

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An experimental study of high heat flux removal by shear-driven liquid films

Intensively evaporating liquid films, moving under the friction of a co-current gas flow in a mini-channel (shear-driven liquid films), are promising for the use in cooling systems of modern semiconductor devices with high local heat release. In this work, the effect of various parameters, such as the liquid and gas flow rates and channel height, on the critical heat flux in the locally heated shear-driven water film has been studied. A record value of the critical heat flux of 1200 W/cm2 has been achieved in experiments. Heat leaks to the substrate and heat losses to the atmosphere in total do not exceed 25% for the heat flux above 400 W/cm2. Comparison of the critical heat fluxes for the shear-driven liquid film and for flow boiling in a minichannel shows that the critical heat flux is an order of magnitude higher for the shear-driven liquid film. This confirms the prospect of using shear-driven liquid films in the modern high-efficient cooling systems

Breakdown dynamics of a horizontal evaporating liquid layer when heated locally

Breakdown of liquid layer when heated from a localized hot spot was investigated experimentally. Water and ethanol were used as working liquids with a layer thickness of 300 μm. Basic steps of the breakdown process were found and mean velocities of the dry spot formation were determined. The formation of residual layer over the hot-spot before the breakdown has been found for both liquids. The creation of a droplet cluster near the heating region is observed when using water as a working fluid. It was shown that evaporation is one of the general factors influencing the process of layer breakdown and dry spot formation as well as thermocapillary effect

Transport and deposition patterns in drying sessile droplets

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106826/1/aic14338.pd