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The presence of surfactants in dried latex films can adversely affect the adhesive, water-resistant, and gloss properties, so investigating the surfactant distribution in latex coatings is of prime industrial relevance. Here we present a model that predicts the distribution of surfactant in a latex coating during the solvent evaporation stage. The conservation equation for surfactant during solvent evaporation is solved in the limit of infinite particle Peclet numbers, a dimensionless quantity giving the measure of relative magnitudes of evaporative to diffusive fluxes. A parametric analysis using the model reveals that the surfactant adsorption isotherm is the determining physical parameter. The model always predicts surfactant excesses at the top surface and either excess or depletion at the bottom surface depending on the isotherm. Uniform distributions are predicted for low surfactant Peclet numbers. Attenuated total reflection Fourier transform infrared spectroscopic probes on film surfaces conform to the behavior predicted by the model.

We use a glass-based microfluidic device to study the electric current behavior of an electrospray process in the presence of a coflowing liquid. The current shows strong voltage dependence and weak flow rate dependence, in stark contrast to classical electrospray. By considering that the current is dominated by convection near the apex of the conical meniscus and driven by tangential electric stresses, we quantitatively capture the voltage and flow rate dependence of the current. Our results elucidate the influence of external field strength and open the way to achieve robust electric control of the current and of the drop size in microfluidics.

The length scales of film thickness non-uniformities, commonly observed in polymer colloid (i.e. latex) films, are predicted. This prediction is achieved by investigating the stability behaviour of drying latex films. A linear stability analysis is performed on a base solution representing a uniformly drying latex film containing a surfactant. The analysis identifies film thickness non-uniformities over two length scales: long (millimetre) range (from lubrication theory) and short (micrometer) range (from nonlubrication theory). Evaporation and surfactant desorption into the bulk film are identified as the primary destabilizing mechanisms during drying. Experimental evidence through direct visualization and atomic force microscopy confirm the existence of non-uniformities over both length scales, which are shown to be functions of parameters such as initial particle volume fraction, surfactant amount and desorption strength, whilst being independent of drying rate.Published in JAIChE 54 (2008) 3092-3105

We apply an electric field to a moderately conducting liquid surrounded by another coflowing liquid, all inside a glass-based microfluidic device, to study nonaxisymmetric instabilities. We find that the bending of the electrified jet results in a steady-state, helicoidal structure with a constant opening angle. Remarkably, the characteristic phase speed of the helicoidal wave only depends on the charge carried by the jet in the helicoidal region and its stability critically depends on the properties of the coflowing liquid. In fact, the steady-state helical structure becomes chaotic when the longest characteristic time is that of the inner liquid rather than that of the outer coflowing liquid. We also perform a numerical analysis to show that the natural preference of the jet is to adopt the conical helix structure observed experimentally.instability | charged jets | electro-coflow | electrospinning A liquid with finite electrical conductivity in the presence of a strong electric field can deform and adopt a conical shape resulting from the balance between electric and surface tension stresses (1). However, near the apex of the cone, this structure is not stable and the associated singularity is replaced by a thin jet (2-6). The resultant cone-jet structure, which is stable within certain values of the applied voltage and imposed liquid flow rate, is the workhorse of electrospray and all its associated applications (7-10).The jet that emanates from this structure always breaks into spherical droplets due to axisymmetric instabilities (11-13). However, in many cases, the jet bends off-axis due to a lateral instability that results from the electrostatic repulsion between bent and straight parts of the jet (14)(15)(16)(17)(18)(19). If the growth rate associated to this whipping instability is larger than that associated to jet breakup, the off-axis movement of the jet becomes the most significant aspect of its evolution. This is exploited in electrospinning, where the simple liquid is replaced by a polymer solution whose solvent evaporates before drop breakup takes place, thus resulting in the formation of polymer fibers (14,17,18,20). The presence of the lateral instability in this application results in thinner fibers, as bending stretches, concomittantly thinning the jet (20). Unfortunately, in most experimental realizations, whipping manifests in a chaotic fashion (15,16,18,19,21,22) preventing us from knowing and unraveling its detailed structure and properties.In this work, we apply an electric field to a moderately conducting liquid surrounded by a coflowing liquid to generate a steady-state whipping structure, which we find is helicoidal, with an amplitude that increases linearly along the downstream direction. Interestingly, the characteristic phase speed of the helical wave is determined by the electrostatic repulsion between the fluid elements of the jet in the whipping region. By performing a numerical analysis, we show that the conical helix structure is the natural configuration of electrified jets,...

Uneven distribution of surfactant in dried latex films can affect the final film properties such as its water-resistance, gloss, and adhesiveness. Therefore, it is important to understand the driving force for surfactant transport during drying. In this paper, the accumulation of surfactant on the surface of poly(styrene-co-butyl acrylate) latex is studied using Rutherford Backscattering (RBS) and compared with results from a model that is based on the diffusive transport of particles and surfactant. Experimentally, a 30-50 nm thick surface layer, rich in surfactant, is seen and the concentration in the bulk of the film, obtained from RBS, agrees, at least qualitatively, with the model predictions for two of the surfactants tested.

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