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Controlling the Dynamics of Liquid Interface

The main focus of the research described in this thesis is the development and analysis of interfacial rheometers. This development involves improvement of the data interpretation of existing interfacial rheometers,as well as the design and experimental validation of some novel approaches. Interfacial rheometry is classified based on the kinematics of the applied deformation into shear, extensional and dilatational rheology. This work contributes to these three fields and additionally, the effect of interfacial rheological properties on the measurement of surface pressure-area isotherms is considered. In the final chapter, the design and fabrication of a microfluidic flow cell aimed to apply a simple bulk shear flow, for example to study the effect of interfacial rheological properties on droplet deformation, is investigated.

For interfacial shear rheology, the derivation of interfacial shear viscosities from experimental results obtained with an Interfacial Stress Rheometer (ISR) is investigated first. More specifically, the effect of subphase drag on the interfacial flow field and the applied shear rates is examined. Using an analytical solution based on a point-force approximation as well as numerical calculations and particle tracking experiments on model interfaces, subphase drag effects are analyzed and quantified. Based on the dimensionless number that governs the extent of this effect, the Boussinesqnumber Bo, it appears that only for Bo>500, subphase drag can be safelyneglected. In other cases, the obtained results should be interpreted using the actual shear rate at the rod, which is higher when subphase drag affects the interfacial flow field.

Next, the measurement of extensional interfacial properties using the Cambridge Interfacial Tensiometer (CIT) is investigated. For this purpose, a constitutive relationship is derived to model a purely viscous interface, characterized by an interfacial shear, dilatational and extensional viscosity. Additionally, surface tension gradients are considered within the model. Numerical calculations are used to characterize the interfacial flow field for different values of the three governing dimensionless groups. The interfacial flow field is a mixed flow field, with contributions from dilatation and extension, the relative balance depending on the interfacial properties.These results imply that in order to interpret the measurement results in terms of extensional viscosities, a model-based approach is a prerequisite. Additionally, data from other interfacial rheometers to quantify the interfacial properties in shear and dilatation are required. Becauseof the direct link between interfacial properties and the interfacial flow field, the sensitivity of the device for the extensional properties remains limited. Moreover, the additional dependence on other measurements makes this technique prone to error propagation. As an experimental test case, experimental data obtained with the CIT device on DPPC layers at the air-water interface are analyzed. Good agreement is found betweenthe measured forces and the model predictions based on literature valuesof the interfacial properties. The results indicate that for DPPC, there is no specific extensional hardening or softening.

Subsequently, a new fixture is designed that, when attached to a standard rotationalrheometer, can probe the dilatational resistance of complex fluid-fluidinterfaces. The fixture itself is an adaptation of the Double Wall-Ringgeometry (DWR), which was designed to measure interfacial shear properties. By adding an undulation to the ring, the obtained fixture, referredto as the Double Wall-Sinusoidal Ring (DWSR), is examined for its ability to probe dilatational rheology of interfaces. Numerical calculations are performed in order to derive the effect of the dilatational properties on the measured torque values as recorded by the rheometer. Like in the CIT device, the induced flow field is mixed, with shear and dilatation the limiting cases here. Nevertheless, the DWSR geometry is found sensitive to the dilatational properties, however, only in a limited range of interfacial characteristics. More specifically, the ratio of dilatational over shear viscosity should be in the range of 20 to 2000 for sufficient sensitivity. For lower values, the torque is dominated by the shearcontribution, whereas for higher values, the interface behaves almost incompressible and the sensitivity is lost. To validate the concept, experiments on hexadecanol, a simple straight-chain alcohol, were performed at the air-water interface. Good consistency was found between the DWSR results and data obtained with another dilatational rheometry technique,the Langmuir trough with oscillating barriers. Despite the limited sensitivity, the DWSR fixture has some advantages over existing dilatationalrheometers. First of all, the shear contributions can be fully corrected for by a model-based approach. Second, the use of a planar interface avoids the contribution of bending elasticity to the induced stresses. Finally, the in-plane torque measurement by a rheometer gives a direct mechanical readout of the stresses and does not rely on image fitting or other indirect stress measurements.

Addionally, the contribution ofrheological effects to the measurement of surface pressure-area isotherms is considered. Surface pressure-area isotherms establish the relationship between the surface pressure and the surface concentration of stabilizer. However, recent reports in literature present isotherm measurements that seem to violate the direct relationship between surface pressureand surface concentration. More specifically, for the same value of surface concentration, different apparent values of the surface pressure are recorded when using different Langmuir trough geometries. These observations are rationalized in this work, by considering the rheological contributions to the apparent surface pressure in the case of highly elastic interfaces. For this purpose, quasi-linear constitutive models for elastic interfaces are developed by using the appropriate finite strain stress tensors, adequately separating the shear and dilatational effects ina frame invariant matter. Using these models, the surface pressure-areaisotherms for elastic interfaces can be interpreted as composed of bothsurface tension and surface stress. A case study of layers of graphene oxide sheets at the water-air interface is performed, and the major partof the recorded isotherm is actually originating from the rheological response rather than the evolution of surface tension. The finite strain tensor that works best to describe the experimental data of the compression isotherms is the Hencky strain tensor. A new measurement protocol isdeveloped where isotherms obtained by spreading different initial surface concentrations on the same interfacial area are measured. This way, by comparing the difference between the recorded isotherms, the rheological and surface tension contributions can be effectively separated.

Because of the limited measuring range of the DWSR, a radial Langmuir trough was developed and validated in order to obtain purely dilatational deformations at a planar interface, avoiding the need for shear contribution corrections. The interfacial deformation is obtained by pulling an elastic barrier isotropically using twelve fingers connected by wires and pulleys to a single stepper motor. The interfacial stress is recorded by a Wilhelmy rod positioned at the center of the trough. This way, both isotherms and small amplitude area oscillations can be measured. Numerical calculations are performed to investigate the effect of subphase drag on the applied deformations and the measuring limits in terms of theBoussinesq number are derived. As an experimental test case, a polymer,PtBMA, was measured at the air-water interface. The apparent isotherms obtained with the radial trough are compared with isotherms obtained from the rectangular trough with different orientations of the Wilhelmy plate. At elevated surface pressures, the shear elasticity of the interfacial layer is sufficiently strong to cause significant deviations in the recorded isotherms from the different trough geometries. This can be observed by interfacial shear measurements using a DWR device at different surface concentrations. When applying small amplitude area oscillations, moduli consistent with the slope of the isotherm are observed for valuesof the surface pressure below 18 mN/m and results are similar for both trough geometries and plate orientations. However, for higher values of the surface pressure, the dilatational moduli are considerably larger than the isotherm slopes, indicating morphological transitions within the interfacial layer. Additionally, deviations between trough geometries and plate orientations are observed, with similar trends as the apparent isotherms, caused by the large values of shear elasticity. The viscous contribution to the dilatational response appears negligible for the studied frequency range.

In the final part, a new design for a microfluidic shear flow cell is optimized and experimentally validated. In order to study the effect of interfacial stabilizers on critical processes in foams and emulsions, such as coalescence and breakup, the ability to apply well-defined flows to droplets or bubbles with controlled interfacial properties is crucial. Therefore, a microfluidic cell is designed to use a simple geometry with only two in- and two outlets in order to create a stagnation point flow with a simple shear region. The microfluidic design was manufactured using photolitography techniques and the PDMS flow cell was studied using confocal microscopy in order to validate the numerical calculations of the flow profiles around the stagnation point.

In summary, this work contributes to the development of interfacial shear, extensional and dilatational rheometry. Both numerical calculations on existing setups as well as the development and experimental validation of new designs are performed. Additional insights regarding the relative importance of the different material functions of complex fluid-fluid interfaces resulted. The interpretation of surface pressure-area isotherms is considered, where rheological contributions can be significant in the case of highly elastic interfaces. Finally, a microfluidic shear flow cell geometry is optimized and implemented that allows a systematic study of the effect of interfacial rheological properties on coalescence and breakup. 
Date:25 Aug 2009  →  7 Oct 2013
Keywords:Interfacial rheology, Coalescence, Droplet deformation, Microfluidics
Disciplines:Process engineering, Polymeric materials, Ceramic and glass materials, Materials science and engineering, Semiconductor materials, Other materials engineering, Physical chemistry
Project type:PhD project