Experimental studies of contact angle hysteresis phenomena on polymer surfaces ­ Toward the understanding and control of wettability for different applications.

Contact angle hysteresis phenomena on polymer surfaces have been studied by contact angle measurements using sessile liquid droplets and captive air bubbles in conjunction with a drop shape method known as Axisymmetric Drop Shape Analysis - Profile (ADSA-P). In addition, commercially available sessile drop goniometer techniques were used. The polymer surfaces were characterized with respect to their surface structure (morphology, roughness, swelling) and surface chemistry (elemental surface composition, acid-base characteristics) by scanning electron microscopy (SEM), scanning force microscopy (SFM), ellipsometry, X-ray photoelectron spectroscopy (XPS) and streaming potential measurements. Heterogeneous polymer surfaces with controlled roughness and chemical composition were prepared by different routes using plasma etching and subsequent dip coating or grafting of polymer brushes, anodic oxidation of aluminium substrates coated with thin polymer films, deposition techniques to create regular patterned and rough fractal surfaces from core-shell particles, and block copolymers. To reveal the effects of swelling and reorientation at the solid/liquid interface contact angle hysteresis phenomena on polyimide surfaces, cellulose membranes, and thermo-responsive hydrogels have been studied. The effect of different solutes in the liquid (electrolytes, surfactants) and their impact on contact angle hysteresis were characterized for solid polymers without and with ionizable functional surface groups in aqueous electrolyte solutions of different ion concentrations and pH and for photoresist surfaces in cationic aqueous surfactant solutions. The work is an attempt toward the understanding of contact angle hysteresis phenomena on polymer surfaces aimed at the control of wettability for different applications.

Microfluidic emulsion separation-simultaneous separation and sensing by multilayer nanofilm structures.

Emulsion separation is of high relevance for filtration applications, liquid-liquid-partitioning of biomolecules like proteins and recovery of products from droplet microreactors. Selective interaction of various components of an emulsion with substrates is used to design microfluidic flow chambers for efficient separation of emulsions into their individual components. Our lab-on-a-chip device consists of an emulsion separation cell with an integrated silicon sensor chip, the latter allowing the detection of liquid motion via the field-effect signal. Thus, within our lab-on-a-chip device, emulsions can be separated while the separation process is monitored simultaneously. For emulsion separation a surface energy step gradient, namely a sharp interface between the hydrophobic and hydrophilic parts of the separation chamber, is used. The key component of the lab-on-a-chip system is a multilayer and multifunctional nanofilm structure which not only provides the surface energy step gradient for emulsion separation but also constitutes the functional parts of the field-effect transistors. The proof-of-principle was performed using a model emulsion consisting of immiscible aqueous and organic solvent components. Droplet coalescence was identified as a key aspect influencing the separation process, with quite different effects during separation on open surfaces as compared to slit geometry. For a detailed description of this observation, an analytical model was derived and lattice Boltzmann computer simulations were performed. By use of grazing incidence small angle x-ray scattering (GISAXS) interfacial nanostructures during gold nanoparticle deposition in a flow field were probed to demonstrate the potential of GISAXS for in situ investigations during flow.

Structure and flow of droplets on solid surfaces.

The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid-solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro- and nanometer-sized beams, GISAXS and in situ imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed in situ during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed.

Stability and dynamics of droplets on patterned substrates: insights from experiments and lattice Boltzmann simulations.

The stability and dynamics of droplets on solid substrates are studied both theoretically and via experiments. Focusing on our recent achievements within the DFG-priority program 1164 (Nano- and Microfluidics), we first consider the case of (large) droplets on the so-called gradient substrates. Here the term gradient refers to both a change of wettability (chemical gradient) or topography (roughness gradient). While the motion of a droplet on a perfectly flat substrate upon the action of a chemical gradient appears to be a natural consequence of the considered situation, we show that the behavior of a droplet on a gradient of topography is less obvious. Nevertheless, if care is taken in the choice of the topographic patterns (in order to reduce hysteresis effects), a motion may be observed. Interestingly, in this case, simple scaling arguments adequately account for the dependence of the droplet velocity on the roughness gradient (Moradi et al 2010 Europhys. Lett. 89 26006). Another issue addressed in this paper is the behavior of droplets on hydrophobic substrates with a periodic arrangement of square shaped pillars. Here, it is possible to propose an analytically solvable model for the case where the droplet size becomes comparable to the roughness scale (Gross et al 2009 Europhys. Lett. 88 26002). Two important predictions of the model are highlighted here. (i) There exists a state with a finite penetration depth, distinct from the full wetting (Wenzel) and suspended (Cassie-Baxter, CB) states. (ii) Upon quasi-static evaporation, a droplet initially on the top of the pillars (CB state) undergoes a transition to this new state with a finite penetration depth but then (upon further evaporation) climbs up the pillars and goes back to the CB state again. These predictions are confirmed via independent numerical simulations. Moreover, we also address the fundamental issue of the internal droplet dynamics and the terminal center of mass velocity on a flat substrate.

In vitro blood compatibility of polymeric biomaterials through covalent immobilization of an amidine derivative.

We present a surface coating with anticoagulant characteristics showing significantly reduced coagulation activation. The synthesis of a monomeric conjugate containing a benzamidine moiety was carried out and its inhibitory activity against human thrombin, the key enzyme of the blood coagulation cascade, was determined using a chromogenic assay. Based on that, low-thrombogenic interfaces were prepared by covalent attachment of this low-molecular weight thrombin inhibitor on poly(octadecene-alt-maleic anhydride) copolymer thin films and characterized using ellipsometry, XPS and dynamic contact angle measurements. The in vitro hemocompatibility tests using freshly drawn human whole blood showed, in agreement with the SEM images, that a PO-MA film modified with a benzamidine moiety using a PEG spacer decreased the activation of coagulation, platelets and the complement system. The decreased protein adsorption, in addition to the specific inhibition of thrombin, effectively enhanced the short-term hemocompatibility characteristics.