Tadashi Kajiya

Controlling the drying and film formation processes of polymer solution droplets with addition of small amount of surfactants.

We studied how the addition of surfactants alters the drying and film formation processes of polymer solution droplets with contact lines strongly fixed by bank structures. We found that even if the amount of surfactant is quite small, it drastically changes the final profile of the polymer film from a ringlike profile to a flat profile. This property is observed commonly, irrespective of the polymer concentration, droplet volume, and type of solvent. We conjecture that the inhomogeneous distribution of the surfactant caused by the outward capillary flow induces the Marangoni flow directed toward the center of the droplet, which suppresses the outward flow. The present phenomenon implies an effective method for controlling the profile of the polymer film in inkjet printing technologies.

Advancing liquid contact line on visco-elastic gel substrates: stick-slip vs. continuous motions

We studied the dynamics of water sessile droplets advancing on hydrophobic and visco-elastic poly(styrene-butadiene-styrene)(SBS)–paraffin gel substrates at various inflation rates. During the advancing process, the droplet contact line exhibits three different regimes of motions. When the contact line advances at a high velocity, it moves continuously with a constant contact angle. As the contact line slows down, it starts a stick-slip motion: the contact line is pinned at a certain position and then suddenly slips forward. With further decrease of the velocity, the contact line stops the stick-slip motion and continuously advances again. The observed threshold values for the transitions of the contact line motions (continuous–stick-slip–continuous) indicate that the rheology of the gel drastically affects the dynamics of liquid on its surface. We suggest that on visco-elastic gels, the moving contact line exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on the characteristic frequency of the gel surface deformation. At an intermediate regime, the stick-slip motion of the contact line appears. We also propose a simple geometrical model in the stick-slip regime which allows us to relate the jumps of the droplet radius to the jumps of the apparent contact angle.

Contact-line recession leaving a macroscopic polymer film in the drying droplets of water-poly(N,N-dimethylacrylamide) (PDMA) solution.

We found that the drying process of the droplet of water-poly(N,N-dimethylacrylamide) (PDMA) solution on a glass substrate shows a somewhat unusual behavior. In this system, the contact line starts to recede at an early stage of drying, and as it recedes, it leaves a macroscopic polymer film behind. The resulting film has a volcano-like profile, but the peak is not located at the edge of the film but in the middle of the film. We studied the drying process changing the polymer concentration and the wetting property of the substrate. We found that the onset of the contact-line recession depends upon the initial contact angle greatly, while the receding contact angle does not depend upon the initial contact angle. We conjecture that this phenomenon is caused by the Marangoni force, which acts to bring the surface of the solution inward because of the negative dependence of the surface tension on the polymer concentration.

Film Formation Kinetics in the Drying Process of Polymer Solution Enclosed by Bank

The drying of a thin film of oligomeric polymer solution placed on a substrate enclosed by a bank is studied by confocal microscopy. It is found that the final polymer film has a crater near the center, but the depth of the crater varies considerably depending on the initial polymer concentration and the initial volume of the droplet. The observation of the time evolution of the thickness profile reveals that the final shape is determined by the process of solidification: as the solvent evaporates, the solidified region starts near the edge of the droplets and propagates toward the center. The relationship between the thickness profile and the initial polymer concentration and the droplet volume is explained qualitatively.

Controlling Profiles of Polymer Dots by Switching between Evaporation and Condensation

We found that the profiles of the dots formed from the drying droplets of polymer solution can be modified by switching between the evaporation and condensation processes. When a polymer dot is exposed to solvent vapor during a certain time and is dried again, the dot profile changes from ringlike to flat. To obtain a flat dot, there exists an optimal exposure time. We conjecture that the change of the dot profile is due to the refluidization of the polymer film. Our results imply a new possibility for controlling the dot profile in inkjet printing technologies.

3D Imaging of Water-Drop Condensation on Hydrophobic and Hydrophilic Lubricant-Impregnated Surfaces.

Condensation of water from the atmosphere on a solid surface is an ubiquitous phenomenon in nature and has diverse technological applications, e.g. in heat and mass transfer. We investigated the condensation kinetics of water drops on a lubricant-impregnated surface, i.e., a micropillar array impregnated with a non-volatile ionic liquid. Growing and coalescing drops were imaged in 3D using a laser scanning confocal microscope equipped with a temperature and humidity control. Different stages of condensation can be discriminated. On a lubricant-impregnated hydrophobic micropillar array these are: (1) Nucleation on the lubricant surface. (2) Regular alignment of water drops between micropillars and formation of a three-phase contact line on a bottom of the substrate. (3) Deformation and bridging by coalescence which eventually leads to a detachment of the drops from the bottom substrate. The drop-substrate contact does not result in breakdown of the slippery behaviour. Contrary, on a lubricant-impregnated hydrophilic micropillar array, the condensed water drops replace the lubricant. Consequently, the surface loses its slippery property. Our results demonstrate that a Wenzel-like to Cassie transition, required to maintain the facile removal of condensed water drops, can be induced by well-chosen surface hydrophobicity.

Dynamics of the contact line in wetting and diffusing processes of water droplets on hydrogel (PAMPS–PAAM) substrates

We studied the dynamics of the wetting and diffusing processes of water droplets on hydrogel (Poly(2-acrylamido-2-methyl-propane-sulfonic acid -co- acrylamide) (PAMPS–PAAM)) substrates. The profiles of the droplet and substrate were measured simultaneously using a grid projection method. We observed that as the water droplet diffuses into the gel, the contact line of the droplet exhibits successively two different behaviors: pinned and receding, and the transition between these two behaviors is closely related to the local deformation of the gel substrate. The contact line is pinned at an early stage. As the water diffusion proceeds, the contact angle of the droplet decreases while the angle of the local slope of the gel surface near the contact line increases. At the moment where these two angles almost correspond to each other, the contact line starts to recede. Our results indicate that due to the water diffusion, a locally swollen region is formed in the vicinity of the droplet–gel interface, and whether the contact line is pinned or recedes is determined by the surface property of this swollen region.

Effects of Gelation on the Evaporation Rate of Polymer Solutions

Effects of elasticity on the evaporation rate of polymer solutions and gels are examined by measuring it for various states of mixtures, that is, monomer solutions, polymer solutions, and gels. For chemical gels the elastic effect on the evaporation rate is observed. For physical gels the elastic effect does not appear due to the finite time relaxation of the network stress. An enhancement of the evaporation rate for the physical gel is observed due to the hydration effect.

A liquid contact line receding on a soft gel surface: dip-coating geometry investigation

We studied the dynamics of a liquid contact line receding on a hydrophobic soft gel (SBS-paraffin). In order to realize a well-defined geometry with an accurate control of velocity, a dip-coating setup was implemented. Provided that the elastic modulus is small enough, a significant deformation takes place near the contact line, which in turn drastically influences the wetting behaviour. Depending on the translation velocity of the substrate, the contact line exhibits different regimes of motions. Continuous motions are observed at high and low velocities, meanwhile two types of stick-slip motion - periodic and erratic - appear at intermediate velocities. We suggest that the observed transitions could be explained in terms of the competition between different frequencies, i.e., the frequency of the strain field variation induced by the contact line motion and the crossover frequency of the gel related to the material relaxation. Our results provide systematic views on how the wetting of liquid is modified by the rheological properties of a complex soft substrate.

Measurement of dynamic surface tension by mechanically vibrated sessile droplets

We developed a novel method for measuring the dynamic surface tension of liquids using mechanically vibrated sessile droplets. Under continuous mechanical vibration, the shape of the deformed droplet was fitted by numerical analysis, taking into account the force balance at the drop surface and the momentum equation. The surface tension was determined by optimizing four parameters: the surface tension, the droplets height, the radius of the droplet-substrate contact area, and the horizontal symmetrical position of the droplet. The accuracy and repeatability of the proposed method were confirmed using drops of distilled water as well as viscous aqueous glycerol solutions. The vibration frequency had no influence on surface tension in the case of pure liquids. However, for water-soluble surfactant solutions, the dynamic surface tension gradually increased with vibration frequency, which was particularly notable for low surfactant concentrations slightly below the critical micelle concentration. This frequency dependence resulted from the competition of two mechanisms at the drop surface: local surface deformation and surfactant transport towards the newly generated surface.