Influence of Sedimentation on Convective Instabilities in Colloidal Suspensions

We investigate theoretically the bifurcation scenario for colloidal suspensions subject to a vertical temperature gradient taking into account the effect of sedimentation. In contrast to molecular binary mixtures, here the thermal relaxation time is much shorter than that for concentration fluctuations. This allows for differently prepared ground states, where a concentration profile due to sedimentation and/or the Soret effect has been established or not. This gives rise to different linear instability behaviors, which are manifest in the temporal evolution into the final, generally stationary convective state. In a certain range above a rather high barometric number there is a coexistence between the quiescent state and the stationary convective one, allowing for a hysteretic scenario.Comment: to appear in Int. J. Bif. Chao

Diffusion in liquid mixtures

The understanding of transport and mixing in fluids in the presence and in the absence of external fields and reactions represents a challenging topic of strategic relevance for space exploration. Indeed, mixing and transport of components in a fluid are especially important during long-term space missions where fuels, food and other materials, needed for the sustainability of long space travels, must be processed under microgravity conditions. So far, the processes of transport and mixing have been investigated mainly at the macroscopic and microscopic scale. Their investigation at the mesoscopic scale is becoming increasingly important for the understanding of mass transfer in confined systems, such as porous media, biological systems and microfluidic systems. Microgravity conditions will provide the opportunity to analyze the effect of external fields and reactions on optimizing mixing and transport in the absence of the convective flows induced by buoyancy on Earth. This would be of great practical applicative relevance to handle complex fluids under microgravity conditions for the processing of materials in space

Convection in colloidal suspensions with particle-concentration-dependent viscosity

The onset of thermal convection in a horizontal layer of a colloidal suspension is investigated in terms of a continuum model for binary-fluid mixtures where the viscosity depends on the local concentration of colloidal particles. With an increasing difference between the viscosity at the warmer and the colder boundary the threshold of convection is reduced in the range of positive values of the separation ratio psi with the onset of stationary convection as well as in the range of negative values of psi with an oscillatory Hopf bifurcation. Additionally the convection rolls are shifted downwards with respect to the center of the horizontal layer for stationary convection (psi>0) and upwards for the Hopf bifurcation (psi<0).Comment: 8 pages, 6 figures, submitted to European Physical Journal

Behavior of thin colloidal films and fundamental studies of convective deposition

Colloidal assembly is emerging in different avenues of modern technology like photonics, membrane, quantum dots, and molecular adsorption devices. Fabricating network of colloidal particle is governed by the surface energy and fluid flows. As you go smaller in size, fundamental understanding of the process is crucial. Previous research in our group was focused on the different applications of using the particle networks. Fabricating these structures on a desired substrate, with a choice of particles, and a desirable number of layers is a tedious task. It demands the thorough understanding of process parameters and different interactions in the thin colloidal film. In the following work, we have separated the convective deposition into two parts. One is governed by thin film stability of solvent, particle interactions with substrate and interface, and interfacial properties, on the other hand, the second part is governed by the solvent flow through a porous structure of particles network. Separating one process into two bits helps in probing for more details. Previous models of convective deposition capture the essential physics over narrow ranges of parameters, however, there is much room for developing robust models over broader deposition conditions and understanding instabilities in this system. In the convective deposition, the suspension flow is driven by solvent evaporation. The widely used Nagayama equation for the convective deposition defines the evaporation loss as the product of the evaporation rate (Je) and drying length ( ). Here, is the hydrodynamic length scale, which for many years’ have been assumed as a constant. Even though it is safe to assume a constant value for in a small region of substrate velocity, it fails to predict the coating thickness over wide velocity range. Following the work done by Y. Jung et al. we have derived an analytical expression for the drying length in the convective deposition for a more general geometry, treating a system as a Darcy flow. This analysis allows the prediction of coating thickness over a wider range of substrate velocities. In depositing a thicker or smaller particles colloidal crystals, the evaporation driven hydrodynamic stress in the thin film causes crack formation. In such cases, the crack spacing varies with the thickness. This phenomenon is not new and has been observed in all length scales. It has been shown that, in a colloidal crystal, a crack spacing scales up with a hydrodynamic stress. Interestingly, the simultaneous 1D drying and particle deposition in convective deposition results in highly linear uniform cracks. We have shown that the crack spacing can be easily tuned with the deposition speed or the substrate temperature. These fundamental studies enable optimizing deposition conditions to produce various thin film structures for a given particle size, solvent, and stabilizing agent. As is done in many previous studies of convective deposition and evaporating droplets, an addition of a surfactant can significantly change the mode of deposition to be nearly independent of the evaporation rate and deposition velocity by altering this length scale and thin film flow. At lower surfactant concentrations, the added surfactant had no effects on the assembly. At higher surfactant concentrations above the critical micelle concentration, the marangoni stresses become the main driving force for the flow inside the thin film. This Marangoni flow which can be much stronger than that driven by the evaporation may be tailored to produce the desired particle depositions over a wide range of velocities. The direction of marangoni flow is important. We have studied the mixture of water-ethanol as a choice of solvent, which allows completely reverse results that of the excess surfactant concentration. We also have studied the particle-particle and particle-substrate interactions, which play a crucial role in the convective deposition. Following results demonstrates the non-trivial effects of varying surface charge and ionic strength of monosized silica microspheres in the water on the quality of the deposited monolayer. The increase in particle surface charge results in a broader range of parameters that result in monolayer deposition which can be explained considering the particle-substrate electrostatic repulsion in solution. Resulting changes in the coating morphology and microstructure at different solution conditions were observed using confocal microscopy enabling correlation of order to disorder transitions with relative particle stability. The following work discusses the novel way of isolating the inherent streaking instabilities happening in the convective deposition. With hydrophobic treating substrate in periodic areas allows one to separate these instabilities, leaving an ideal coating in between. The final part of this thesis is devoted to a relatively new scale-up technique of continuous particle assembly. We have studied the limitations on microstructure at higher substrate velocities as well as a smaller improvement in microstructure using a bidisperse silica suspension

Investigations into Convective Deposition from Fundamental and Application-Driven Perspectives

Crystalline particle coatings can provide critical enhancement to wide-ranging energy and biomedical device applications. One method by which ordered particle arrays can be assembled is convective deposition. In convective deposition, particles flow to a surface via evaporation-driven convection, then order through capillary interactions. This thesis will serve to investigate convective deposition from fundamental and application-driven perspectives. Motivations for this work include the development of point-of-care diagnostic devices, macroporous membranes, and various energy applications. Immunoaffinity cell capture devices display enhanced diagnostic capabilities with intelligently varied surface roughness in the form of particle coatings. Relatedly, highly crystalline particle coatings can be used to template the fabrication of macroporous polymer membranes. These membranes display highly monodisperse pores at particle contact points. In addition, ordered areas of particles, acting as microlenses, can enhance LED performance by 2.66-fold and DSSC efficiency by 30%. Previous research has targeted the formation of crystalline monolayers of particles. However, much insight can be gleaned from imperfect coatings. The analysis of submonolayer coatings, exhibiting significant void spaces, provides insight as to the specific mechanisms and timescales for flow and crystallization. A pair of competing deposition modes, termed ballistic and locally-ordered, enables the intelligent design of experiments and enables significant enhancement in control of resultant thin film morphology. Surface tension-driven particle assembly is subject to a number of native instabilities and macroscale defects that can irreversibly compromise coating uniformity. These include the formation of three-dimensional streaks, where surface tension-driven flow spurs on the nucleation of large imperfections. These imperfections, once nucleated, exhibit a feedback loop of dramatically enhanced evaporation and resultant flow. In addition, thick nanoparticle coatings, subject to enormous drying stresses, exhibit highly uniform crack formation and spacing in an attempt to minimize system energy. Both these imperfections yield insight on convective deposition as a fundamental phenomenon, and intelligent design of experiments moving forward. Cracking can be suppressed through layer-by-layer particle assembly, whereas streaking can be controlled via several significant process enhancements. Process enhancements include the addition of smaller constituent, as packing aids, to suspension, the application of lateral vibration, and the reversal of relevant surface tension gradients. The transition from unary to binary suspensions represents a significant improvement to convective deposition as a process. Nanoparticles act as packing, and flow, aids, wholly suppress macroscale defects under ideal conditions. A relative deficiency or excess of nanoparticles can generate complex coating morphologies including multilayers and transverse stripes. The application of lateral vibration to convective deposition allows the assembly of monolayer particle coatings under a larger range of operating conditions and at a faster rate. Macroscale defect formation can increased through an enhancement of the natural condition, where evaporative cooling generates a thermal gradient in drying droplets. Conversely, these defects can be suppressed with a reversal of this gradient, which will reverse the direction of surface tension-driven recirculation. These fundamental developments in understanding, and associated process enhancements, are critical in current efforts to scale up convective deposition. As convective deposition evolves from laboratory-scale batch experiments to continuous, large scale, coatings, repeatability and robustness, as well as an ability to controllably change thin film morphology, will be essential