Structuring of polymer surface by evaporation of sessile microdrops [Elektronische Ressource] / Guangfen Li

Li Feng

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2007
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Structuring of polymer surface by
evaporation of sessile microdrops



DISSERTATION
zur Erlangung des Grades "Doktor der Naturwissenschaften"
am Fachbereich Chemie/Pharmazie
der Johannes Gutenberg-Universität
in Mainz





vorgelegt von
Master-Chem. Guangfen Li
aus Tianjin (V. R. China)

Mainz – 2007






















Die vorliegende Arbeit wurde unter Betreung von
Herrn Prof. Dr. Hans-Jürgen Butt im Zeitraum
zwischen April 2004 bis März 2007 am
Max-Plank-Institut für Polymerforschung, Mainz,
Deutschland angefertigt.

IIABSTRACT
ABSTRACT
In this thesis different homemade experimental setups were used to study (1) the
evaporation dynamics of liquid mcirodrops on smooth, insoluble planar surfaces, (2) the
microstructuring of soluble polymer surfaces by solvent drops, and (3) the interfacial
tension between two immiscible slugs in a microcapillary, where evaporation is avoided.
(1) The evaporation dynamics of pure water drops on self-assembled monolayers of thiols
and disulfides (SAMs) on gold showed a stronger pinning of the rim of the droplet, the more
hydrophilic the surface was. The hydrophilicity was tuned by the end-groups of the SAM
alkyl chains. The total evaporation time, t , for drops with different initial volumes, V , tot 0
bobeys a power law of the type V = a⋅t with an exponent b differing from the theoretical 0 tot
expected value of 1.5 for a diffusion-limited evaporation. This is explained with an
increased evaporation through a thin water layer in contact with the droplet.
(2) If a pendant toluene drop is deposited from a syringe tip on a soluble polystyrene (PS)
surface by fast up- and downward motions of the substrate table (~ 9 mm/s), a concave
microtopology is observed after drop evaporation. This can be explained by a flow of
solvent and dissolved polymer to the rim during the evaporation of the pinned solvent drop,
a phenomenon known from ring-like coffee-stains. When the retraction speed is decreased
to 10 µm/s, polymer dissolution is favored and a convex structure is observed instead. This
can be explained with a gelation of the polymer-rich droplet. Concave microstructures
occurred even if the pendant drop was evaporating close to the polymer surface without
contact. This can be attributed to a diffusion of toluene into PS.
Additionally, the dissolution rate of the polymer is decreasing with increasing molar mass.
Thus, the microtoplogy changes from convex (gelation-driven), over concave (flow-driven),
to disordered pile-ups (instability-driven). For the system polyethylmetha-
crylate/ethylacetate exclusively concave structures occured, most likely due to the lower
dissolution and/or faster evaporation rate.
IIIABSTRACT
(3) The interfacial tensions between slugs of water and different organic solvents in a
microcapillary were successfully measured with a novel homemade technique, especially
useful for the application in microfluidics.
























IVINDEX
INDEX
ABSTRACT ................................................................................................................. III
INDEX............................................................................................................................V
1 INTRODUCTION AND MOTIVATION ............................................................8
2 FUNDAMENTAL ................................................................................................14
2.1 SESSILE DROPLETS ..........................................................................................14
2.1.1 Young’s equation for sessile droplets ....................................................14
2.1.2 Evaporation dynamics of sessile drops ..................................................15
2.1.2.1. Spherical geometry.........................................................................15
2.1.2.2. Evaporation mode and evaporation rate of sessile drops ...............17
2.1.3 Coffee ring effect and Marangoni effect................................................20
2.2 POLYMER DISSOLUTION BEHAVIOR OF POLYMER.............................................24
2.2.1 Processes in sessile evaporating solvent droplets on a polymer surface 25
2.2.2 Mechanism of polymer dissolution........................................................25
2.2.3 Dependence of polymer dissolution rate on disentanglement and solvent
diffusion ................................................................................................................28
2.2.4 Instabilities during and after the evaporation process............................30
2.3 DETERMINATION OF INTERFACIAL TENSION ....................................................31
3 EXPERIMENTS...................................................................................................34
3.1 CHEMICAL AND MATERIALS ...........................................................................34
3.1.1 Cleaning of glassware/substrates ...........................................................34
3.1.2 Self-assembled monolayer (SAMs) .......................................................34
3.1.2.1. Chemicals.......................................................................................34
3.1.2.2. Synthesis of asymmetric dialkyl disulfides SAMs.........................35
3.1.2.3. Preparation of self assembled monolayer on gold-mica substrate .36
3.1.3 Polymer substrates and drop evaporation...............................................38
3.1.3.1. Chemicals38
3.1.3.2. Preparation of polymer substrate....................................................38
3.1.3.3. Preparation of OTS-silicon substrate .............................................40
3.1.4 Microchannel experiments.....................................................................41
3.1.4.1. Chemicals41
3.1.4.2. Consumable materials ....................................................................41
3.1.4.3. Preparation of glass capillary and chemical treatment...................42
3.1.5 Optical parts...........................................................................................42
3.2 METHODS OF INVESTIGATION..........................................................................43
3.2.1 Self-made double camera system...........................................................43
3.2.1.1. Experimental setup for evaporation of droplets on SAMs.............43
3.2.1.2. ental setup for evaporation of solvent drop on polymer
substrates ........................................................................................................44
VINDEX
3.2.1.3. Droplets in microchannels - microfluidics setup ........................... 46
3.2.2 Laser scanning confocal microscopy..................................................... 49
3.2.3 3D confocal white light surface microscopy ......................................... 51
3.2.4 Scanning electron microscopy ............................................................... 52
3.2.5 Tensiometer............................................................................................52
4 RESULT AND DISCUSSION ............................................................................ 54
4.1 EVAPORATION DYNAMICS OF PURE WATER DROPLETS ON SELF ASSEMBLED
MONOLAYERS.............................................................................................................. 54
4.1.1 One droplet experiment..........................................................................54
4.1.1.1. Evaporation mode in dependence of surface hydrophilicity.......... 54
4.1.1.2. Advancing contact angle in dependence of surface hydrophilicity56
2/3 4.1.1.3. Dependence of drop volume and volume on evaporation time .. 58
4.1.1.4. Dependence of drop cap radius on evaporation time..................... 59
4.1.2 Multi-drop evaporation dynamics..........................................................60
4.1.2.1. Volume changes with total evaporation time................................. 61
4.1.2.2. b in dependence of surface properties............................................ 63
4.1.3 Investigation of evaporation dynamics of pure water droplets on SAMs64
4.1.3.1. Static contact angle measurement.................................................. 64
4.1.3.2. The influence of surface roughness and heterogeneity on
evaporation dynamics ........................................................................................ 65
4.1.3.3. Drop size influences on the evaporation dynamics........................ 66
4.1.3.4. Influences of pined three phase line on the evaporation dynamics 67
4.1.3.5. The influence of droplet cooling effects on exponent b values ..... 70
4.1.3.6. The influence of the surface chemical compositions on evaporation
dynamics ........................................................................................................72
4.2 MICROSTRUCTURE FORMATION OF POLYMER SURFACES BY DEPOSITION OF
SOLVENT DROPLETS .............................