Evaporation of sessile microdrops studied with microcantilevers [Elektronische Ressource] / vorgelegt von Dmytro S. Golovko

Description

Evaporation of Sessile Microdrops studied with Microcantilevers DISSERTATION zur Erlangung des Grades "Doktor der Naturwissenschaften" im Promotionsfach Physikalische Chemie am Fachbereich Chemie, Pharmazie und Geowissenschaften der Johannes Gutenberg-Universität Mainz vorgelegt von Master-Chem. Dmytro S. Golovko geb. in Slavyansk (Ukraine) Mainz, 2008 Dekan: 1. Berichterstatter: 2. Berichterstatter: Tag der mündlichen Prüfung: April, 15, 2008 ii Die vorliegende Arbeit wurde unter Betreuung von Herrn Prof. Dr. Hans-Jürgen Butt im Zeitraum zwischen Oktober 2005 bis Dezember 2007 am Max-Planck-Institut für Polymerforschung, Mainz, Deutschland angefertigt. iiiIndex Index iv Introduction and Motivation 1 Chapter 1. Fundamentals 4 1.1Young’s equation 1.2 Evaporation Dynamics of Sessile Drops 6 1.2.1 Modes of Evaporation 6 1.2.2 Evaporation Rate and Time for Sessile Microdrops 7 1.2.3 Surface Cooling due to Evaporation 12 1.3 Sessile Drop on a Cantilever 14 1.3.1 Drop at the Free End of the Cantilever 14 1.3.2 Drop in the Middle of the Cantilever 18 Chapter 2. Materials and Methods 24 2.1 Instrumentation 24 2.1.1 Reversed Particle Interaction Apparatus 24 2.1.2 Regimes of R-PIA Operation 28 2.1.

Sujets

• Physik

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2008
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

   

Evaporation of Sessile Microdrops studied with Microcantilevers         DISSERTATION   zur Erlangung des Grades "Doktor der Naturwissenschaften" im Promotionsfach Physikalische Chemie   am Fachbereich Chemie, Pharmazie und Geowissenschaften der Johannes Gutenberg-Universität Mainz        vorgelegt von Master-Chem. Dmytro S. Golovko geb. in Slavyansk (Ukraine)         Mainz, 2008  

       

 

 

 

Dekan:

1. Berichterstatter: 2. Berichterstatter:   Tag der mündlichen Prüfung: April, 15, 2008

ii

                                     

     

 

                                            Die vorliegende Arbeit wurde unter Betreuung von Herrn Prof. Dr. Hans-Jürgen Butt im Zeitraum zwischen Oktober 2005 bis Dezember 2007 am Max-Planck-Institut für Polymerforschung, Mainz, Deutschland angefertigt.

iii

Index           Introduction and Motivation           Chapter 1. Fundamentals 1.1Young’s equation 1.2 Evaporation Dynamics of Sessile Drops  1.2.1 Modes of Evaporation  1.2.2 Evaporation Rate and Time for Sessile Microdrops  1.2.3 Surface Cooling due to Evaporation 1.3 Sessile Drop on a Cantilever  1.3.1 Drop at the Free End of the Cantilever      1.3.2 Drop in the Middle of the Cantilever

  Chapter 2.Materials and Methods 2.1 Instrumentation 2.1.1 Reversed Particle Interaction Apparatus 2.1.2 Regimes of R-PIA Operation 2.1.3 Piezoelectric Drop Generator 2.1.4 Experiments in Saturated Atmosphere 2.2 Cantilevers 2.3 Chemicals and Cantilever Surface Preparation 2.4 Cantilever Calibration 2.5 Drop-On-Cantilever Simulations      2.5.1 Spring Constant of a Cantilever 2.5.2 Evaporative Cooling Effect

                       

  Chapter 3. Results and Discussions 3.1 Cantilever Spring Constant Calibration using Water Microdrops 3.1.1 Constant Drop Location 3.1.2 Constant Drop Mass  iv

   

                 

                       

       

Index 

iv 1

4 4 6 6 7 12 14 14 18

24 24 24 28 28 30 31 32 32 34 34 35

37 38 39 41

3.1.3 Conclusions 3.2 Evaporation of Microdrops from Atomic Force  Microscope Cantilevers 3.2.1 Evaporation of Water and n-Alcohols from Hydrophobic Cantilevers 3.2.2 Cooling Effect of Evaporating Water Drops 3.2.3 A Method for Surface Stress Evaluation during Drop Evaporation 3.2.3.1 Technique Validation     3.2.3.2 Negative Inclination and Surface Stress  Evaluation 3.2.4 Evaporation of Drops in Saturated Atmosphere 3.2.4.1 Experiment 3.2.4.2 Conclusions

  Summary and Conclusions References        List of Abbreviations Acknowledgements    

 

   

   

   

   

   

v

   

   

   

   

   

   

                   

   

   

                   

   

   

44 45 45 50 54 55 60 64 64 66

67 69

76  77  

Introduction and Motivation

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      Evaporation of drops of liquids freely flying in air or sitting on a solid is a very common dynamic phenomenon which can be seen in everyday life. Evaporating drops tune many natural phenomena like clouds formation and rain fall, appearance of rainbow and fogs, thus being a crucial inseparable part of the gigantic atmospheric thermal and mass circulation. At the human scale, a number of technological and industrial processes depends severely on drop evaporation phenomena. Among them are heat transfer applications [1-4], the combustion of fuel, prevention degradation of coatings, ink-jet printing [5-7], surface modification [8-11] and various applications in microfluidics [12-15]. The rate of evaporation of free suspended drops depends mainly on the nature of the liquid and on some physical parameters of its surroundings, like temperature and pressure. The rate of evaporation of sessile drops is additionally determined by the surface properties of solids, liquids and gases, and therefore its study can give a key to better understanding interfacial interactions. Moreover, processes occurring in a drop or on the solid surface can affect the evaporation rate and thus can be investigated. It is already more that 100 years ago as Maxwell proposed the consideration of the evaporation rate as governed by the diffusion of molecules in the gas phase [16]. Since then, many refinements have been proposed to better describe the basic modes of evaporation of sessile drops depending on the wetting properties of the substrate [17-29]. These studies were mostly dealing with a quantitative description of either a change of the contact angle and contact area monitored by video systems [23, 27, 28, 30] or a change of mass monitored by a quartz crystal microbalance (QCM) [31, 32]. The smallest drop size analyzed was on the order of 10μm or several nanograms. An intriguing question is the dynamics of drop evaporation in its very last stages, when, for example, the Laplace pressure significantly increases, the vapor pressure according to the Kelvin equation increases, and surface effects play a predominant role. Neither the QCM nor video microscopy are suitable for tracking the evaporation to its very end, because the drops are too small. Recently, a nanomechanical resonator [33] was designed to monitor in time the mass variation of evaporating liquid droplets. The authors are capable of tracing the drop mass down to several femtoliters.

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However, the question on the influence of surface effects on the last stages of the drop evaporation was not yet addressed. By now, evaporation of sessile microdrops can be successfully accomplished on atomic force microscope (AFM) cantilevers [34, 35] due to their high sensitivity and possibility to partly trace the evaporation dynamics of small drops at high time resolution (below a millisecond). The drop causes the cantilever to bend, since the elastic response of thin solids is comparable with forces exerted by the drop over the three phase contact line and its contact area [36-39]. The Laplace pressure acting over the drop contact area and the surface tension pulling the drop at its rim are not enough to cause noticeable bending of a hard and thick solid (see schematics in Fig. 1A), while a hard and thin plate can be bent by typically a few hundreds of nanometers (see Fig. 1B). In general, when a drop is deposited on a cantilever several types of measurements can be performed: spring constant calibration of the cantilever (see Fig. 1C); sensing of drop evaporation by surface forces (see Fig. 1D); sensing drop evaporation by mass change; sensing of the heat exchanged during evaporation.  In this work, I present and discuss an extension to the technique based on the use of AFM cantilevers to trace the evaporation dynamics of drops by two means simultaneously. This allows, first, to monitor cantilever bending caused by surface tension and Laplace pressure inside the drop, second, to follow mass reduction due to the shift in resonance frequency during drop evaporation. The technique is assessed by tracing the evaporation dynamics of water drops from hydrophobic cantilevers. From both signals, cantilever bending and resonance frequency, drop parameters can be extracted. The technique is used to evaluate quantitatively the cantilever overbending arising at the last stages of the evaporation of water drops on hydrophilic cantilevers. In turn, I show how to calibrate AFM cantilevers loaded by drops [34] of different masses presenting an extension to the well-known added-mass method [40] for the spring constant calibration of AFM cantilevers [41]. The evaporation rate of microscopic drops is higher than that of macroscopic ones due to their highly curved surface. As a result, they have a higher vapor pressure at the liquid/gaseous interface than planar liquid surfaces or large drops. In this case nonequilibrium effects take place [42-45]. Thus the applicability of Youngs equation for microdrops thus needed to be verified. Therefore I performed a series of experiments on evaporation of small drops in nearly saturated atmosphere. Conclusion on the significance of Youngs equation for microdrops are drawn [46].      

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  Figure 1. Drop on thick (A) and thin (B) solids. Two kinds of measurements on a cantilever: spring constant calibration (C), and evaporation tracing of small drops (D).                            

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3