Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 107

Contents
ListofAbbreviationsandSymbols .................................... 105

A. Introduction ............................................... 107

B. Background, Materials and Methods . . . .............................. 108

1. Drop in Equilibrium ........................................ 108

2.EvaporatingDrop ......................................... 109

3. Deformable Surfaces . . . .................................... 110

4. Evaporation of Solvent Droplets on Polymer Surfaces ..................... 111

C.ApplicationsofEvaporatingSolventDropsonDeformable/SolubleSubstrates .......... 114

1. Microlenses on Polymer Surfaces by ink jetting]Ink Jetting Solvent Drops .......... 114

2. Microstructures on Polymer Surfaces by mixtures of solvents]Mixtures of Solvents . .... 115

3. Microlithography and Other Applications ............................ 116

D.DecouplingtheProcesses ....................................... 119

1.MassTransportInsidetheDrop:theCoffeeStainEffect.................... 120

2. Dissolution and Swelling of the Substrate ............................ 121

3.EtchingandCollapseoftheSubstrate.............................. 122

4.ElasticDeformationoftheSubstrate............................... 122

E.BriefSummary ............................................. 124

F.Acknowledgements........................................... 125

G.References................................................ 125

A. Introduction
The evaporation of drops has developed lately to an extremely active and prolific
area of research, since it is relevant for a vast number of technological applications.
Processes such as wetting, dewetting or drying occur in a many industrial processes,
like coatings, crop spraying, printing, or biotechnological applications. Evaporation
of microdrops is also of interest in the study and development of heat transfer de-
vices for cooling microelectronic devices, or in combustion technologies. The use
of droplets as microreactors in nanochemistry or as liquid media for DNA optical
mapping as well demands a detailed knowledge of the evaporating mechanism of
sessile drops. In fundamental research, the study of evaporation drops is used in
order to understand the properties of interfaces between solids, liquids and gases.
The first theory of the evaporation of a free spherical liquid drop in an infinitely
extending surrounding gas was proposed by Maxwell more than a century ago [1].
Assuming that evaporation is only limited by diffusion of the vapor molecules away
from the drop, the rate of mass change could be well described [2, 3]. The same the-
oretical approach also applies to sessile drops. Since then, the evaporation of large
drops from solid surfaces is by now widely studied and understood. Picknett and
Bexon [4] proposed evaporation models for sessile drops observing that there are
two principal modes of evaporation: one where the contact area of the drop remains
constant while the contact angle decreases, and the other one where the contact area
diminishes with a constant contact angle. They also observed a mixed mode close to
the end of evaporation, where the mode changes from one to the other in the course
of evaporation. Shanahan and Bourges [5] suggested that a sessile drop in open-air
conditions evaporates in three stages: In the first one, the contact radius of the drop