Convective mass transport dominates surfactant adsorption in a microfluidic Y-junction.

Surfactant adsorption during emulsification can be quantified by measuring the acting interfacial tension using a Y-junction microfluidic device. To obtain insight into the surfactant transport mechanism to the interface, the effect of shear force on the acting interfacial tension was assessed by systematically varying the continuous phase viscosity and velocity. Varying the continuous phase viscosity did not affect the acting interfacial tension, indicating that surfactant adsorption during Y-junction emulsification is not diffusion-limited. The acting interfacial tension was inversely dependent on the continuous phase velocity, which indicates that surfactant adsorption is governed by convective mass transfer resulting from the continuous phase velocity. The acting interfacial tension can be measured in the sub-millisecond time scale and under convective transport conditions using the Y-junction. These conditions are relevant to industrial emulsification and cannot be assessed by conventional tensiometry techniques (e.g., drop tensiometers) where surfactant adsorption is mostly driven by diffusion. We believe, therefore, that this method can be used to understand emulsifier adsorption during industrial emulsification, which can, in turn, be used to rationally design emulsion formulations and processes.

Using FT-IR as a fast method to measure fatty acid soaps in in vivo and in vitro digests.

The objective of the current study was to develop a simple method to measure fatty acid soaps, making use of FT-IR, representative for the soap formation observed in clinical trials. Calcium soaps have a unique coordination which leads to a typical double-splitting of the antisymmetric and symmetric carboxylate peaks. Absorbance values of these carboxylate peaks were used together with the absorbance of the hydrocarbon -CH2 antisymmetric and symmetric peaks to calculate the calcium soap absorbance. Based on the linear correlation between the calcium soap absorbance and the calcium soap concentration measured with GC-FID, a model was set-up and subsequently successfully validated to quantify calcium soap concentrations in faecal samples from clinical trials with this FT-IR method. With in vivo as well as in vitro digestion an inverse correlation between the long chain saturated fatty acid part of milk fat containing fat blends used for the infant formulas, and the formation of fatty acid soaps after digestion and defaecation could be observed. There is a clear link between the amount of long chain saturated fatty acids at the sn-1/3 position and their release as free fatty acid after lipolysis with the appearance of fatty acid soaps. These insights enable future development of fat blends for infant nutrition to optimize fatty acid soap formation and thereby gut discomfort in infants. These insights can be used to predict the soap formation capacity of a newly designed fat blend and thereby the improvement of infant nutrition products.

The effect of dissolved gas on coalescence of oil drops studied with microfluidics.

HYPOTHESIS: In literature it is stated that the stability of oil-in-water emulsions could be enhanced by decreasing the so-called "hydrophobic interactions" between surfaces through removal of dissolved atmospheric gases. Since the effect of the dissolved gases depends on the hydrophobicity of the oil phase, as well as the system pressure, we vary this effect systematically and monitor droplet coalescence in a tailor-made microfluidic device. EXPERIMENTS: The coalescence of oil drops in standard and degassed conditions was studied by direct observation using a microfluidic setup. Two model oils (heptane and xylene) were used to represent different hydrophobicity of the dispersed phases, together with an oil with dynamic interfacial behaviour (diluted crude oil). In addition, the effect of the volume fraction, droplet size and degassing method was studied. FINDINGS: At ambient pressure, the degassing of the continuous phase reduced the extent of coalescence for the model oils, which is in agreement with other reports. No effect of the dissolved gases was found on the drop formation process. At elevated pressures, the dissolved gases influenced only the most hydrophobic oil (heptane), while causing no effect in the other systems. The coalescence frequencies decreased upon the reduction of the drop sizes, which was justified with the theory for two interacting spheres.

Interfacial tension measured at high expansion rates and within milliseconds using microfluidics.

To understand droplet formation and stabilisation, technologies are needed to measure interfacial tension at micrometer range and millisecond scale. In this paper, microtechnology is used, and that allows us to access these ranges and derive a model for surfactant free systems. The predicting power of the model was tested, and we found that it can be used to accurately (validated with >60 experiments) describe droplet size for a wide range of flow rates, interfacial tensions, and continuous phase viscosities. The model was used next to determine interfacial tensions in a system with hexadecane and sodium dodecylsulfate (SDS) solutions, and it was found that the model can be used for droplet formation times ranging from 0.4 to 9.4ms while using a wide range of process conditions. The method described here differs greatly from standard dynamic interfacial tension methods that use quiescent, mostly diffusion-limited situations. The effects that we measured are much faster due to enhanced mass transfer; this allows us to assess the typical time scales used in industrial emulsification devices.