Langmuir 2009, 25, 75-78
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Transition in the Evaporation Kinetics of Water Microdrops on Hydrophilic Surfaces Dmytro S. Golovko, Hans-Ju¨rgen Butt, and Elmar Bonaccurso* Max-Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed October 19, 2008. ReVised Manuscript ReceiVed NoVember 23, 2008 We describe a technique that allows measurement of the mass and shape of sessile liquid microdrops during evaporation. Therefore, the microdrops are deposited by an inkjet onto a silicon microcantilever, and the bending and the shift in resonance frequency are monitored. From hydrophobized surfaces, microscopic water drops evaporate with the same kinetics as macroscopic drops; we verify the validity of known evaporation laws to drops with diameters from 100 µm to below 10 µm. From hydrophilic surfaces, the evaporation is slowed down during the last ∼100 ms; we believe that this occurs due to flattening of the drops, which are then stabilized by interfacial forces and disjoining pressure.
The first theory of the evaporation of a free spherical liquid drop in an infinitely extending surrounding gas was proposed by Maxwell more that 100 years ago.1 Assuming that evaporation is limited by diffusion of the vapor molecules away from the drop, the rate of mass change could successfully be described.2,3 The same theoretical approach also applies to sessile drops. Two basic evaporation modes for macroscopic drops have been devised: A sessile drop evaporates with a constant contact angle Θ (CCA) and decreasing contact radius, or it evaporates with a constant contact radius a (CCR) and decreasing contact angle. For water drops, the first mode was observed mainly on hydrophobic and the second on hydrophilic surfaces. A mixed mode, where angle and contact radius change, can also occur toward the end of evaporation. The mode of evaporation determines the evaporation laws, that is, the change of drop mass (or volume) vs time. For CCA evaporation and for Θ e 90°, the mass of the drop m to the power of 2/3 decreases linearly with time: m02/3 - m2/3 ∝ t. Here, m0 is the initial mass at t ) 0.4,5 For CCR evaporation, a linear decrease of the mass according to m0 - m ∝ t has been observed in some cases.6-9 Other experiments confirm the m2/3 ∝ t law, regardless of whether the evaporation mode is CCA, CCR, or mixed.10,11 Several other studies relate properties of the substrate (e.g., wettability, roughness, thermal conductivity) to the evaporation of the liquid.12-17 * Corresponding author.
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While the evaporation of macroscopic liquid drops is wellunderstood and experimentally verified, the models for the evaporation of microdrops (diameter
