Joachim Venzmer

Dynamic Wetting of Hydrophobic Polymers by Aqueous Surfactant and Superspreader Solutions

In this paper, we comparatively investigated the wetting performance of aqueous surfactant solutions in a wide range of concentrations, including conventional ionic surfactants (CTAB, SDS) and two nonionic polyether-modified trisiloxane surfactants (TSS6/3, TSS10/2), over hydrophobic polypropylene substrates. In all cases, scaling analysis of the experimental data of spreading drops showed that the early spreading stage was dominated by inertia and that the duration of this stage was not influenced by the addition of surfactant. For conventional surfactant solutions, we only observed the inertia-dominated spreading stage before the drops stopped wetting with a finite stable contact angle. For both trisiloxane surfactants, after the inertial stage we observed a second viscosity-dominated spreading stage. In this stage, TSS10/2 showed an enhanced wetting capability independent of its concentration, while TSS6/3 started to show a concentration-dependent spreading behavior that was fully developed in a third superspreading stage. Our findings suggest that the superspreading property of TSS6/3 began to take effect after a characteristic time, before which the superspreading TSS6/3 and the nonsuperspreading TSS10/2 behaved similarly. Power law fits to the superspreading regime are in agreement with an interpretation of Marangoni flows resulting from surface tension gradients.

Imaging internal flows in a drying sessile polymer dispersion drop using Spectral Radar Optical Coherence Tomography (SR-OCT)

In this work, we present the visualization of the internal flows in a drying sessile polymer dispersion drop on hydrophilic and hydrophobic surfaces with Spectral Radar Optical Coherence Tomography (SR-OCT). We have found that surface features such as the initial contact angle and pinning of the contact line, play a crucial role on the flow direction and final shape of the dried drop. Moreover, imaging through selection of vertical slices using optical coherence tomography offers a feasible alternative compared to imaging through selection of narrow horizontal slices using confocal microscopy for turbid, barely transparent fluids.

Intact deposition of cationic vesicles on anionic cellulose fibers: Role of vesicle size, polydispersity, and substrate roughness studied via streaming potential measurements.

HYPOTHESIS Understanding the mechanism of intact vesicle deposition on solid surfaces is important for effective utilization of vesicles as active ingredient carriers in applications such as drug delivery and fabric softening. In this study, the deposition of large (davg=12μm) and small (davg=0.27μm) cationic vesicles of ditallowethylester dimethylammonium chloride (DEEDMAC) on smooth and rough anionic cellulose fibers is investigated. EXPERIMENTS The deposition process is studied quantitatively using streaming potential measurements and spectrophotometric determination of DEEDMAC concentrations. Natural and regenerated cellulose fibers, namely cotton and viscose, having rough and smooth surfaces, respectively, are used as adsorbents. Equilibrium deposition data and profiles of substrate streaming potential variation with deposition are used to gain insights into the fate of vesicles upon deposition and the deposition mechanism. FINDINGS Intact deposition of DEEDMAC vesicles is ascertained based on streaming potential variation with deposition in the form of characteristic saturating profiles which symbolize particle-like deposition. The same is also confirmed by confocal fluorescence microscopy. Substrate roughness is found to considerably influence the deposition mechanism which, in a novel application of electrokinetic methods, is elucidated via streaming potential measurements.

Surfactant-Enhanced Spreading of Sessile Water Drops on Polypropylene Surfaces

Spreading of water drops resting in equilibrium on polypropylene surfaces was initiated by dispensing surfactant-laden droplets on their apex. Upon contact of the two drops two processes were kicked-off: surfactant from the droplets spread along the water/air interface of the sessile drops and a train of capillary waves propagated along the sessile drops. The contact line of the sessile drops remained initially pinned and started spreading only when surfactant reached it while the capillary waves did not have an apparent effect on initiating drop spreading. However, surfactant influenced the propagation velocity of the capillary waves. Though the spreading dynamics of such nonhomogeneously mixed surfactant/water drops on polypropylene surfaces was initially different from that of homogeneously mixed drops, the later spreading dynamics was similar and was dominated by viscosity and surface tension in both cases. These results can help in discriminating the path of action of surfactants in bulk and at the water/air interface, which is also relevant for understanding phenomena such as superspreading.

Electrokinetic investigation of deposition of cationic fabric softener vesicles on anionic porous cotton fabrics

HYPOTHESIS Colloidal deposition on porous substrates is a complex process influenced by both, (i) characteristics of colloidal permeation into porous substrates, and (ii) mechanism of colloidal deposition on solid surfaces. Such processes are quintessential to action of products such as hair conditioners and fabric softeners where the substrates to be treated are porous. The performance of these formulations is linked with the distribution of deposited colloids across porous substrates i.e. whether deposition is localized near substrate periphery, or deposition is homogeneously distributed. EXPERIMENTS In this work, we investigate the deposition of cationic vesicles, commonly used in fabric softeners, on anionic porous cotton yarns via spectrophotometric measurement of adsorption density of vesicles on yarns and electrokinetic measurement of cotton yarn apparent zeta potentials. Under the employed conditions, cotton yarn apparent zeta potentials are sensitive predominantly to external yarn surfaces. Therefore, these measurements can distinguish between deposition on external and internal yarn surfaces. FINDINGS The phase behavior of lipid bilayers constituting the vesicles is identified as an important governing factor with solid-gel vesicles depositing more near yarn periphery, and liquid-crystalline vesicles depositing more uniformly throughout the yarns. Bulk electrical conductivity also influences the distribution of deposited vesicles. The results are explained with the help of a newly proposed theory.

Effect of Geometry on Electrokinetic Characterization of Solid Surfaces.

An analytical approach is presented to describe pressure-driven streaming current (Istr) and streaming potential (Ustr) generation in geometrically complex samples, for which the classical Helmholtz-Smoluchowski (H-S) equation is known to be inaccurate. The new approach is valid under the same prerequisite conditions that are used for the development of the H-S equation, that is, the electrical double layers (EDLs) are sufficiently thin and surface conductivity and electroviscous effects are negligible. The analytical methodology is developed using linear velocity profiles to describe liquid flow inside of EDLs and using simplifying approximations to describe macroscopic flow. At first, a general expression is obtained to describe the Istr generated in different cross sections of an arbitrarily shaped sample. Thereafter, assuming that the generated Ustr varies only along the pressure-gradient direction, an expression describing the variation of generated Ustr along the sample length is obtained. These expressions describing Istr and Ustr generation constitute the theoretical foundation of this work, which is first applied to a set of three nonuniform cross-sectional capillaries and thereafter to a square array of cylindrical fibers (model porous media) for both parallel and transverse fiber orientation cases. Although analytical solutions cannot be obtained for real porous substrates because of their random structure, the new theory provides useful insights into the effect of important factors such as fiber orientation, sample porosity, and sample dimensions. The solutions obtained for the model porous media are used to device strategies for more accurate zeta potential determination of porous fiber plugs. The new approach could be thus useful in resolving the long-standing problem of sample geometry dependence of zeta potential measurements.

Chapter 6 – Superspreading

Superspreading is a fascinating phenomenon that was first observed more than 20 years ago with dilute solutions of trisiloxane surfactants on hydrophobic substrates. It has attracted much attention mainly for two reasons: (i) the practical benefits of the effect in agrochemical applications and (ii) the scientific challenge to explain both the physics behind the phenomenon and the mode of action of the surfactants. Despite the work of many groups all over the world that has contributed significantly to the understanding of this phenomenon, the reasons why only some trisiloxane surfactants promote superspreading, whereas others of similar chemical structure behave more like ordinary surfactants, is still somewhat of a mystery. Therefore, it will continue to be an active field of research in interface science to elucidate the exact role of the surfactants in the superspreading process. This chapter not only covers the different approaches to explain superspreading, but also discusses the general, more philosophical aspects of these attempts; basically, it is all about how to discover the truth about an—on a molecular scale—unobservable phenomenon.