Miniaturized cell and flow reactor for QCM-D measurements on fluids and soft matter

Sammanfattning: Thin soft matter films are frequently used in many technical applications, where phenomena taking place in the interfacial regions, control film properties and functionality. In order to optimize the latter there is a clear need of methods and tools for probing diverse physical properties of thin soft matter films confined in a nano-sized region. The latter implies a requirement to control the interfacial properties by a proper choice of surface treatment (chemical or/and physical). It is difficult to control thin soft matter films or liquids, confined in a small volume, because of ubiquitous processes like evaporation, wetting/dewetting and interfacial anchoring, etc. The aim of the present work was to develop an experimental platform for overcoming these obstacles in a sensing context. We concentrated on developing a nanocell and miniaturized flow reactor suitable for operating in conjunction with a QCM-D sensor. Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) is a frequently used sensor for characterizing viscoelastic properties of soft matter materials. This is a main reason why we have chosen it as the initial sensing platform in the present work. The two devices developed here consist of a substrate (the QCM-D sensor) and a lid that are kept separated by a fabricated wall. The latter represents a spacer structure, defining a cell or a reaction chamber. The cell or flow reactor thus has a well-defined height, down to the nanometer range, and a minimal volume in the sub-nanoliter regime. The unique feature of the proposed and tested devices, compared to common microfluidic devices, is that the radial extension can remain macroscopic and that the inner surfaces (substrate and lid) of the cell/reactor can be prepared with a well-defined physics or chemistry before the parts are assembled, since no etching is required. The extreme radius to height ratio makes surface induced effects significant and even dominant in some cases. This in turn makes the measurements of interfacial properties in thin, confined, soft matter films accessible via different sensing platforms. The two devices are a nanocell and a miniaturized flow reactor. The basic difference between them lies in the operating mode. In the former case the device is used in the static batch mode while in the latter case one operates in the flow mode with feeding channels connected to either the lid or integrated in the spacer wall structure for facilitating loading and unloading of sample material. As an application the behaviour of a nanocell containing a (semi)static volume of an isotropic liquid (ethylene glycol) is characterized theoretically and experimentally. Thin liquid ethylene glycol films were confined in a cell with a thickness ranging from bulk (600 mm) down to 50 nm and their viscoelastic properties were probed using shear acoustic waves in the 5-35 MHz range. We studied how the coupling of the acoustic signal, generated by the QCM-D sensor, interacts with the confining lid of the nanocell, and how this affects the resonant frequency and dissipation values of the system. This type of results is important for extraction of viscoelastic properties of the confined liquid from the measured frequency and dissipation curves. We found that it is possible to probe confined films down to 50 nm and that the sensor-lid coupling causes a very strong increase in dissipation factor as the film thickness decreases below the extinction depth of an acoustic shear wave in the confined medium. At the same time the frequency shift decreases and changes sign for some overtones in case of very thin films (<100 nm). The same device was also used for probing the viscoelastic response of 5CB liquid crystal thin films, in the vicinity of the nematic-isotropic phase transition. In the latter experiments well defined anchoring conditions, in the nematic phase on both interfaces, were established to illustrate the versatility of the nanocell. Also in this case the nanocell showed the same general trend as for ethylene glycol, i.e., increasing dissipation and decreasing resonant frequency as the confined films became thinner. In addition, we show how the nanocell can be used to distinguish the viscoelastic behaviour of 5CB confined films when the molecular axis is oriented parallel with and perpendicular to the shearing direction in the nematic phase. In the isotropic phase the viscoelastic properties of the films were identical as expected. As a final application example, the concept was used in a fluidic system for detecting the formation of a lipid bilayer on a silicon oxide surface. In this example the spacer height was larger (ca 45 ?m) and the volume ca 3 ?l, which still is about one order of magnitude smaller than the commercial QCM-D instrument cell used for comparison. The bilayer formation was studied at different buffer flow rates. We find that when scaling the flow rate to the volume of the respective chambers (nanocell respective commercial one), the time for formation of a complete bilayer is about the same. Furthermore, theoretical estimates show that the size of the diffusion zones close to the interfaces, compared to the central flow region in the miniaturized chamber, is substantially larger than for the larger chamber. This implies that a larger fraction of the sample molecules are available for attachment to the sensor surface.