Job offers

PhD students and a PostDocs to work on various issues of microdrop evaporation and dynamic wetting on complex surfaces (see also Research pages)

The Max Planck Institute offers its PhD students a broad range of experimental methods, techniques, and specialized courses. The good technical equipment and the intense supervision allow obtaining a PhD within a relatively short time-frame. You will work in an international environment and in collaboration with international partners.

We expect you to have finished your university studies in physics, chemistry, engineering or a neighbouring field with an above-average Master, German Diplom, or equivalent. We are looking for experimentalists with a strong sense for theoretical questions and interdisciplinary research.

 

Project Nr. 1:

Evaporation of drops or microdrops of mixtures, suspensions and dispersions on microstructured and soft surfaces

Contact persons: Elmar Bonaccurso

Research Areas: evaporation kinetics, capillary forces, interfacial forces, hydrodynamics

Project Description: Evaporation of pure liquids from solid surfaces is well understood. Less well studied is the evaporation of mixtures, of suspensions or dispersions from solid surfaces, or the evaporation of liquids from microstructured or soft surfaces. During evaporation of a mixture the ratio of the two (or more) components continuously changes, thus changing the partial vapour pressures and the surface tension. This can have a big effect on the overall evaporation time. Similarly, when the liquid phase of a dispersion or of a suspension evaporates, the solid phase enriches. This affects evaporation as well. Further, microstructures influence the movement of the rim of the drop during evaporation, and thus its evaporation kinetics.  Similarly do soft surfaces: drops can deform the surface to various extents, and this deformation mostly hinders the movement of the drop rim.

We will analyse and model the parameters which play a role for such systems, and we will try to find the underlying laws governing the evaporation of mixtures, suspensions and dispersions. The findings can have direct applications for the printing, the automotive and the painting industry.

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Project Nr. 2:

Impact and evaporation of microdrops of solvents on soluble polymer surfaces

Contact persons: Elmar Bonaccurso

Research Areas: drop impact dynamics, hydrodynamics, evaporation kinetics, capillary forces, polymer physics

Project Description: A drop of a solvent should completely wet (zero contact angle) a polymer surface, if it is a good solvent for the polymer. Experimentally it is found, however, that drops can form contact angles as large as 10 degrees on polymer surfaces. This phenomenon needs to be quantitatively understood. Qualitatively, when the polymer is dissolved in the solvent, the surface tension of the liquid changes, and so does its wetting efficiency. From experiments we know that the wetting efficiency is changed already after a few milliseconds, whereas the solvation of polymer into a solvent is believed to be a slower process.

We plan to do high-speed impact experiments of drops on soluble surfaces to study the time scale of the processes of drop impact, equilibration of the drop contact angle, solvation of polymer, and evaporation of the polymer/solvent solution.

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Project Nr. 3:

Dynamic and forced wetting/dewetting of complex soft surfaces

Contact persons: G?ter K. Auernhammer, Elmar Bonaccurso, Hans-J?gen Butt

Research Areas: dynamic wetting of complex surfaces, capillary forces, hydrodynamics

Project Description: Forced wetting is relevant in a number of technical processes like printing, painting, and surface coatings in general. Wetting/dewetting of simple hard surfaces is well understood, while that of soft and/or complex surfaces is not. Under complex surfaces we understand microstructured or chemically patterned surfaces with a characteristic pattern size below 100 um. Soft surfaces will include solid surfaces coated with viscoelastic films of varying thickness (100 nm - 100 um) and elastic modulus E (1 kPa - 1 MPa).

Measurements with condensing or evaporating water drops have shown a strong dependence of wetting/dewetting dynamics on the viscoelasticity of, for example, soft polydymethylsiloxane films. The condensation and evaporation rates of single drops differ strongly on hard surfaces and soft surfaces of comparable wettability. Also the movement of the three phase contact line (TPCL) is significantly slowed down on soft surfaces. This evidences the prominent role played by the dynamics of the TPCL on soft surfaces.

Existing experimental setups for measurements of contact line dynamics can reliably control the wetting/dewetting speeds from few um/s up to some mm/s. We want to study wetting/dewetting processes starting from the quasi-static case (close to the equilibrium contact angle) up to the regime where hydrodynamic effects become dominant (TPCL speeds from 0.01 mm/s up to 1 m/s). Whereas for the low-speed part of the measurements we plan to use a commercial tensiometer (DCTA 11, DataPhysics Instruments GmbH), we want to develop a new setup capable of realizing speeds beyond 1mm/s. The aim is to acquire both the dynamic contact angle between liquids and solids and the shape of the contact line for the analysis of hydrodynamic processes.