TY - JOUR
T1 - Evaporatively-driven Marangoni instabilities of volatile liquid films spreading on thermally conductive substrates
AU - Kavehpour, Pirouz
AU - Ovryn, Ben
AU - McKinley, Gareth H.
N1 - Funding Information:
The authors would like to thank L. Mahadevan for insightful discussions on the mechanism of instability and S. Chaieb for assistance in some preliminary experimental measurements of spreading liquid films. This research was supported by the National Aeronautics and Space Administration under grant NAG3-2155.
PY - 2002/7/9
Y1 - 2002/7/9
N2 - Laser confocal microscopy is used to non-invasively investigate the steady and unsteady evolution of viscous microdroplets on solid substrates. Three characteristic dynamical regimes of spreading drops (viscous-capillary, viscous-inertia-capillary, and inertia-capillary) are studied using this non-invasive optical technique. It is shown that the dynamics of each regime depend on the Ohnesorge number, Oh=μ/(ρRσ)1/2, and on the relative magnitudes of the droplet height, radius, compared with the capillary length, ℓcap=√σ/ρg. The power-law relationships between the extent of spreading and elapsed time that are extracted from the experiments are in excellent agreement with available analytical results. We also study the onset and evolution of surface instabilities of the slightly volatile liquid films as they spread across the thermally-conductive surfaces. When the fluid droplet is a volatile silicone oil and the surface is a smooth silicon wafer, an evaporatively-driven thermocapillary instability leads to onset of a time-dependent free surface motion. Below a certain critical thickness (∼20 μm), waves can be observed on the free surface of the film, and the confocal technique is used to measure the amplitude, the frequency, and non-linear evolution of these waves. We interpret these waves in terms of evaporatively-driven Marangoni instabilities induced by surface tension gradients close to the moving contact line. Experiments show that the amplitude and the critical onset thickness of the disturbances vary with the viscosity and the volatility of the liquid, and also with the surface roughness and thermal diffusivity of the substrate. The critical onset conditions for this evaporatively driven instability can be characterized by a dimensionless interfacial thermal resistance, R, which has to be larger than a critical value at the onset of instability. We also demonstrate that this evaporatively-driven Marangoni instability can be eliminated by reducing the volatility of the liquid or the thermal diffusivity of the substrate.
AB - Laser confocal microscopy is used to non-invasively investigate the steady and unsteady evolution of viscous microdroplets on solid substrates. Three characteristic dynamical regimes of spreading drops (viscous-capillary, viscous-inertia-capillary, and inertia-capillary) are studied using this non-invasive optical technique. It is shown that the dynamics of each regime depend on the Ohnesorge number, Oh=μ/(ρRσ)1/2, and on the relative magnitudes of the droplet height, radius, compared with the capillary length, ℓcap=√σ/ρg. The power-law relationships between the extent of spreading and elapsed time that are extracted from the experiments are in excellent agreement with available analytical results. We also study the onset and evolution of surface instabilities of the slightly volatile liquid films as they spread across the thermally-conductive surfaces. When the fluid droplet is a volatile silicone oil and the surface is a smooth silicon wafer, an evaporatively-driven thermocapillary instability leads to onset of a time-dependent free surface motion. Below a certain critical thickness (∼20 μm), waves can be observed on the free surface of the film, and the confocal technique is used to measure the amplitude, the frequency, and non-linear evolution of these waves. We interpret these waves in terms of evaporatively-driven Marangoni instabilities induced by surface tension gradients close to the moving contact line. Experiments show that the amplitude and the critical onset thickness of the disturbances vary with the viscosity and the volatility of the liquid, and also with the surface roughness and thermal diffusivity of the substrate. The critical onset conditions for this evaporatively driven instability can be characterized by a dimensionless interfacial thermal resistance, R, which has to be larger than a critical value at the onset of instability. We also demonstrate that this evaporatively-driven Marangoni instability can be eliminated by reducing the volatility of the liquid or the thermal diffusivity of the substrate.
KW - Evaporation
KW - Marangoni instability
KW - Optical techniques
KW - Spreading viscous drop
KW - Volatile films
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U2 - 10.1016/S0927-7757(02)00064-X
DO - 10.1016/S0927-7757(02)00064-X
M3 - Article
AN - SCOPUS:0037046929
SN - 0927-7757
VL - 206
SP - 409
EP - 423
JO - Colloids and Surfaces A: Physicochemical and Engineering Aspects
JF - Colloids and Surfaces A: Physicochemical and Engineering Aspects
IS - 1-3
ER -