Abstract
Magnetic Resonance Imaging, MRI, flow mapping experiments were designed, developed and tested to investigate flow patterns within deformed drops of non-Newtonian polydimethylsiloxane PDMS relaxing to equilibrium shape. The drops have a density of approximately 0.97 g/ml and were made neutrally buoyant in a Newtonian solution of deuterium oxide and deuterated methanol to match the same density. The latter was adjustable depending on the position the drop was required to be in within the solution (e.g. high up or at the bottom). The drops were deformed using gravitational, rotational and reciprocating forces using specially designed electro-mechanical kits. The drops were typically 5mm average diameter and typical velocities were 1mm/s. Drops have a range of viscosities that fall between 1000cSt and 10000cSt and average molecular weights of 28000g/mole and 35000g/mole respectively. They were tested with and without the addition of three different surfactant (i.e. Octaethylene glycol monododecyl ether C28H58O8) concentrations. Three MRI flow data sets were obtained in the form of velocity maps. The first and second sets were acquired just after drop deformation. The relaxation was recorded in the required plane using a Fast Low Angle Shot (FLASH) flow acquisition program. For each drop a movie of ten flow patterns images, with an average of 3 s to 9s in between each image was created to monitor the relaxation process. The results were compared to learn about the effect of viscosity and surfactant concentration on drops' behaviour. The third data set was acquired during the deformation of drops, using a reciprocating force. The deformation was recorded using a gated Pulsed Gradients Spin-Echo (PGSE) flow measurement acquisition program. For each drop flow patterns were formed at six different positions of the reciprocal drop deformation cycle. Both velocity acquisition programs were based on a Phase Contrast PC velocity measurement concept. These resultant flow patterns allowed us to look non-destructively at the internal flow of deformed drops, showing flow activities in real time. These constructed flow patterns demonstrated a wide range of responses, which substantially contributed to drop deformation studies and provided access to unique and rich information that until now was not accessible. The method offers data which will ultimately contribute to the on going theoretical efforts to understand and predict, hence control and improve, the drop's material properties world-wide.