The Rheology and Fluid Mechanics of Oscillatory Squeeze Flow

Rheology is a diverse field with a wide selection of rheometers capable of measuring fluid properties in a variety of flow conditions. Recently, a new type of rheometer operating on the principles of oscillatory squeeze flow has begun to attract some interest. The theory behind oscillatory squeeze flow rheometry has been known for a long time, but due to its similarity to extensional rheometry, extensional strain has conventionally been used to describe squeeze flow deformations. This work investigated the flow fields present in oscillatory squeeze flow so as to better understand the limitations of this type of device, its proposed applications, and the potential for a more appropriate definition of strain in these types of rheometers. Through the use of finite element simulations and laboratory trials, an oscillatory squeeze flow rheometer was shown to be capable of measuring the rheological properties of both Newtonian and viscoelastic fluids, with an accuracy rivalling that of conventional rotational rheometers. Additionally, the ability of this rheometer to take measurements while submerged in a test fluid was also examined, as past theory development has focused on oscillatory squeeze flow of only a fluid layer between two plates. The stress response of a viscoelastic fluid is dependent on the strain to which it is subjected. In the case of squeeze flow, this strain is generally characterised as the amplitude of plate displacement divided by the gap between the two plates. While this is an accurate definition of extensional strain, fluids in squeeze flow are predominantly undergoing shear flow, and the magnitude of this flow is dependent on the plate dimensions in addition to the displacements (gap, amplitude, and frequency) of the plates. As a result, the current definition of strain does not allow for comparison between different rheometers and thus, a new definition for strain was proposed that allows for shear strain comparison between rheometers of different dimensions. This relationship was examined using both computer simulations and laboratory experiments comparing strain sweeps of materials in various rheometers. Ultimately, this work provides a relationship which allows for the characterisation of a fluid's viscoelastic regime in a squeeze flow rheometer in such a way that it can be related to measurements taken on both other squeeze flow rheometers and rotational rheometers. Most challenges encountered when trying to take oscillatory squeeze flow measurements are due to problems with instrument design. The wide range of devices used while collecting data for this work resulted in a unique knowledge of the various approaches to the mechanics of oscillatory squeeze flow. The methods used by each of these devices to control key aspects of the rheometer (plate displacement, gap calibration etc.) were investigated and discussed. From this, a set of design principles for oscillatory squeeze flow rheometers was proposed which takes advantage of the best aspects of each of these devices