Effect of external fields on the dynamics of colloidal phase transitions

Resumen

In this experimental work, the effects of the external fields on the colloidal phase transition from liquid phase to solid deposits (evaporative colloidal phase transition) have been investigated. The external fields are applied while the transition is in process. The experiments are performed with two different transition duration: (a) In the experiments of long duration, the fluid is allowed to evaporate by exposing the colloidal dispersion (negatively charged polystyrene particles of diameter 1.3 μm dispersed in ultra pure water) to an environment at high temperature (63 degree C) and low humidity (below 2% RH). The colloidal dispersion is placed between the two vertical conducting substrates. Electric fields (DC) are of the order of 1 V/mm and they are applied perpendicularly to the substrates while the phase transition is in process. When the continuous phase evaporates, the contact line recedes. We measure the speed of the receding contact line for different initial concentrations (0.1%, 0.3% and 0.5% w/w, respectively) as well as for varying electric field. The dried deposits of colloidal particles are then correlated with the initial conditions and electric field strength of the respective experiment. To deepen the understanding of the three phase contact line in vertical deposition of colloids, the meniscus of a colloid is observed while the weak external field (AC) of the order of 1 V/mm and 1 Hz is applied. In this case, the working temperature is relatively low (room temperature) when compared to the previous set of experiments explained above and consequently the contact line does not recede during the measurement time. The applied field generates flows near the meniscus through electrokinetic and electrowetting mechanisms resulting in the formation of clusters of colloidal particles in the fluid matrix along the horizontal contact line. The clusters are separated by a well defined characteristic length and in our experimental conditions, they remain between 5 and 15 minutes. (b) In the experiments of short duration (spin-coating), the fluid phase of the colloidal dispersion is made to evaporate in fractions of a second by pouring a volume of the dispersion over the spinning substrate (spinning rate is of the order of 1000 rpm). In the absence of the magnetic field, equivalent film thickness for different kinds of colloids (superparamagnetic and nonmagnetic, respectively) are compared. After that, external fields are applied while the dispersion is pipetted onto the spinning substrate. On the one hand, external magnetic fields up to 0.066 T are applied while spin-coating the dilute superparamagnetic colloidal dispersion (polystyrene coated magnetite of diameter 1 to 2 μm and SiO2 coated magnetite of diameter 1.51 μm dispersed in ultra pure water, respectively). A spin-coating model is constructed by considering the evaporation of the fluid and the particulate characteristics of the spin-coated deposits. Morphological transition from sparse to submonolayer deposits (clusters) of superparamagnetic particles occurs. The magnetic field increases the effective viscosity of the dispersion through magnetic dipole interactions. On the other hand, to overcome the axial symmetry imposed by the spin-coating (in experiments with nonmagnetic particles of high initial concentration, 40% w/w), nonuniform alternating electric fields of the order of 0.1 kV/mm; frequency of the order of 1 kHz are applied while spin-coating the colloidal dispersion (SiO2 particles of diameter 458 nm dispersed in 2-Pentanone) over the patterned conducting substrate. We conclude that the dielectrophoretic confinement of the dispersion affects the hydrodynamic flows resulting in a predefined direction for the colloidal deposits. Thus, the electric field breaks the axial symmetry.