OPTMANDROPS · Optical manipulation of droplets and interfaces in colloid-polymer mixtures

Optical manipulation of droplets and interfaces in colloid-polymer mixtures

See attachment for the full summary including images. Text only below. • Summary description of the project objectives Interfacial phenomena, and in particular the influence of interface fluctuations on the transport and the response of fluids, are of key importance for the behaviour of fluids in confinement, which is becoming increasingly relevant given the ongoing miniaturization of lab-on-a-chip devices. Actively controlling these interfacial phenomena is thus very desirable for the development and optimisation of such devices. While colloidal systems are known to be excellent model systems to study the interface dynamics in great detail as the ultra-low interfacial tension (nN/m) leads to an interfacial roughness in the micrometer range, actively controlling interfacial phenomena has not yet been achieved in colloidal systems. In this project, we aim to actively control interfacial phenomena in phase separated colloid-polymer mixtures using optical trapping techniques. The project consists of the following two major research objectives: 1. Optical manipulation of isotropic colloid-polymer mixtures 2. Optical manipulation of liquid crystalline colloid-polymer mixtures • Description of the work performed since the beginning of the project To achieve these objectives, firstly an optical trapping setup was developed in which the trapping laser enters the sample perpendicular to gravity. This allows manipulation of the interface parallel to the direction of gravity, as is schematically depicted in Fig. 1(left). The trapping laser can be controlled with an acousto optic deflector (AOD) or spatial light modulator (SLM) or both simultaneously, and the sample is observed with a home-built fluorescence microscope. Phase separating mixtures of poly(methyl methacrylate) (PMMA) colloids and xanthan polymer were prepared that phase separate into a colloidal liquid and colloidal gas phase (see Fig. 1). The thermal capillary waves of the interface in these mixtures were measured using confocal laser scanning microscopy and analysed using capillary wave theory. These mixtures were then used to manipulate the interface as shown in Fig. 1. By slowly moving the laser beam from the colloidal liquid phase to the colloidal gas phase, the interface can be distorted in a very controlled way and this was exploited to study the static and dynamic response of the interface in detail. Mathematica routines were written to analyse the results and theory was developed to understand and describe the obtained results. Subsequently, anisometric (plate-shaped) colloids were synthesized and samples were prepared that formed nematic and columnar phases. However, due to problems with gelation stable phase separating mixtures with polymer could not be prepared and therefore manipulation the interface in these systems could not be studied. The possibilities of the developed trapping setup were further exploited by studying the evaporation of droplets of colloidal liquid in the colloidal gas phase. • Description of the main results achieved so far We have shown that the developed trapping setup allows for very controlled manipulation of the colloidal gas-liquid interface. The static shape of the distorted interface as well as the relaxation of the interface after the trap is switched off were successfully related to the capillary wave correlation functions through the fluctuation dissipation theorem (FDT). We have also demonstrated that the optical trapping setup can be used for more complicated interface deformations, such as the example shown in Fig. 2(a-c) where an interface is deformed by two traps simultaneously. The robustness of the theoretical description of the distortion of the interfaces using FDT is reflected by the fact that these results can also be perfectly fitted by our theory as shown in Fig. 2(a-c). Fig. 2(d-g) illustrates how the setup can be used to study droplets by snapping them from the interface. Alternatively, the optical trap can be placed in the colloidal gas phase until the particle concentration in the trap is high enough for a drop of colloidal liquid to condensate within in the trap. We showed that the evaporation rate of these droplets after the trap has been switched off depends on the chemical potential difference between the colloidal liquid and colloidal gas phase. • Expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far). We have achieved active control of interfacial phenomena in colloid–polymer mixtures with an ultra-low interfacial tension using optical trapping parallel to the interface. This experimental framework makes it now possible to address a wide variety of interfacial phenomena in great detail. The results will be particularly relevant for strongly confined interfaces, such as the case in nanofluidic devices. The obtained knowledge will be used to improve and optimise such devices.

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