Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
5 results
Search Results
Item Mixed Organic Surfactant Effects on Cloud Condensation Nuclei(2021) Mitchell, Ian Wallace; Asa-Awuku, Akua; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atmospheric aerosols affect Earth’s radiative budget through direct and indirect effects. The direct effects are well understood but the indirect effects have large uncertainty associated with them. Uncertainty is so great that even the sign of the radiative forcing associated with indirect effects is questioned. This work examines aerosol indirect behavior by assessing surfactant effects on the activation of aerosol particles into cloud droplets. Szyszkowski-Langmuir surface tension models are applied to Köhler theory to capture surfactant effects on aerosol activation behavior. Surfactant aerosols tested are succinic acid and sodium dodecyl sulfate (SDS). Results suggest that a small addition of surface active material (like SDS) to organic carboxylic acids (like succinc acid) can significantly change droplet activation behavior.Item Toxicity of Catanionic Vesicles to Biological Cells(2013) Rao, Veena; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates "catanionic" vesicles, which are nanoscale containers that are spontaneously formed by mixing cationic and anionic surfactants. These structures are easy to prepare and indefinitely stable. In comparison, unilamellar liposomes based on phospholipids are cumbersome to prepare, requiring multiple steps and intense shear (extrusion or sonication); moreover, they have limited stability, especially when stored at room temperature. Despite the many advantages, catanionic vesicles are not frequently used in the pharmaceutical industry because of concerns over their cytotoxicity. In this thesis, we systematically explore the cytotoxicity (on mammalian cell lines) of a range of catanionic vesicles formed by mixing various commercially available cationic and anionic surfactants. We examine how cytotoxicity is influenced by the surfactant tail length, the nature of the surfactant tail (saturated vs. unsaturated) and the net charge on the vesicles; as a control, we also study liposomes from phospholipids. A live/dead assay was our primary tool for assessing cytotoxicity. Our results reveal several systematic trends and we have found that certain vesicles based on unsaturated cationic surfactants are relatively nontoxic and biocompatible. These results could potentially lead to new classes of catanionic vesicles that could be safely utilized for biomedical applications.Item The Effect Of Surfactants On The Breakup Of An Axisymmetric Laminar Liquid Jet(2012) Walker, Justin Robert; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The contacting of multiple liquid phases is a complex process, and one that is difficult to study experimentally. Liquid dispersion studies in stirred tanks and high shear mixers frequently involve the use of surfactants without a strong physical understanding of how the surfactants affect the mechanics of droplet production and breakup. In this study, experiments are performed using the axisymmetric laminar jet system. The breakup of a laminar axisymmetric jet is a well-studied fluid dynamics phenomenon. Despite the extensive literature on jet breakup, the impact of surface active agents on jet breakup has received limited attention. An extensive series of experiments with water-air and oil-water jet systems with and without surfactants has been performed, varying fluid flow rate, jet diameter, jet bulk viscosity, surfactant type, and surfactant concentration. Surfactants were found to significantly affect the breakup of laminar liquid jets. Significant effects on both the length of jets and the size of resulting droplets are reported. In general, the effect of surfactants is to reduce the interfacial tension of the system in question, which results in longer jet breakup lengths and larger diameter droplets. However, the interfacial tension alone is insufficient to explain the physics of the jet breakup phenomena. Several breakup mechanisms were identified, and the regimes in which each operates vary not only due to jet geometry and velocity, but on the interfacial properties as well. The effect of surfactants on the breakup phenomena differs in each of these distinct breakup regimes. A mechanistic model for the prediction of breakup length for surfactant laden jets is presented. This model results in good agreement between predicted and experimentally observed values over a wide variety of surfactant concentrations and jet conditions and was shown to be useful for both the oil-water and water-air systems, within the axisymmetric jetting regime.Item Self-Assembled Photoresponsive and Thermoresponsive Fluids with Tunable Rheology(2009) Kumar, Rakesh; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids whose rheological properties can be tuned by light or heat (termed as photorheological (PR) or thermorheological (TR) fluids, respectively) have attracted a lot of attention as they can be useful in numerous applications such as drug delivery, coatings, sensors, and valves for microfluidic devices. However, current formulations of these fluids suffer from several limitations: in particular, they often require synthesis of complex organic molecules by elaborate procedures, and this limits the widespread use of these fluids. In this dissertation, we seek to develop and investigate new classes of PR and TR fluids based on organic molecules that are readily available and quite inexpensive. Since no new synthesis is required, these systems could prove to be more attractive for a variety of applications. In the first part of this study, we describe a new aqueous photorheological (PR) fluid based on the zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB) and the photosensitive molecule, ortho-methoxy cinnamic acid (OMCA). EDAB/OMCA fluids exhibit photogelling, i.e., a large (~ 10,000 fold) increase in viscosity upon exposure to UV radiation. We show that this photogelling is caused by the growth of long wormlike micelles in the sample. This structural change, in turn, is induced by the UV-induced isomerization of OMCA molecules from their trans to cis form. Evidence from zeta-potential studies, small-angle neutron scattering (SANS), and rheology are used to systematically reveal the molecular and microstructural mechanism for our results. In the second part of this study, we turn our attention to non-aqueous solvents and demonstrate a new class of PR fluids using such solvents. The PR effect here relies on transformations of "reverse" micellar structures formed by a well-known lipid (lecithin) in conjunction with para-coumaric acid (PCA). Lecithin/PCA fluids exhibit a substantial decrease in viscosity upon exposure to UV light (i.e., photothinning). Initially, the molecules self-assemble into long wormlike micelles, leading to highly viscoelastic fluids. Upon UV irradiation, PCA is photo-isomerized from trans to cis. This change in geometry induces a transition from long to short micelles. In turn, the solution viscosity is decreased by more than three orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. In the last study, we report a class of aqueous fluids whose viscosity increases upon heating (i.e., thermo-thickening). These fluids are mixtures of telechelic associating polymers (HEURs) and a type of supramolecules called cyclodextrins (CDs) in water. Interestingly, we observe this behavior only with a particular type of CDs, called alpha-CDs, and not with the other common CD types, i.e., beta- and gamma-CDs. These results are explained in terms of a competition between the hydrophobic end-caps and the hydrophilic backbone of the polymer for complexation with alpha-CD molecules. We have also investigated the effect of amphiphiles (single-tailed surfactants and double-tailed lipids) on the thermo-thickening. The addition of lipids substantially enhances the thermo-thickening behavior, which is explained to be due to an enhancement of the connectivity of hydrophobic junctions by lipid vesicles.Item Effect of Surfactants on Drop Size Distributions in a Batch, Rotor-Stator Mixer(2004-12-17) Padron, Gustavo A.; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Surfactants are often required to stabilize liquid-liquid dispersions produced by rotor-stator mixers. Since drops are deformed rapidly due to the high power input of these devices, the dynamic interfacial properties governed by the surfactant adsorption rate have a significant effect on the resulting drop size. The objective of this work is to develop a fundamental link between surfactant adsorption dynamics, interfacial properties, and turbulent emulsification processes in rotor-stator mixers. The mean drop size and drop size distributions (DSD) of dilute dispersions produced by a batch rotor-stator mixer were studied. Silicone oils of various viscosities were dispersed in aqueous nonionic surfactant and aqueous methanol solutions. The aqueous methanol (clean) systems allowed comparison of surfactant-laden to surfactant-free systems with similar equilibrium interfacial tensions. The DSD were measured via a video microscopy/automated image analysis technique. The equilibrium interfacial tension of clean and surfactant systems was measured, via a pendant drop technique, as a function of methanol and surfactant concentration, respectively. The dynamic surface tension of surfactant solutions was similarly measured. By fitting the data to the Langmuir adsorption isotherm and a long time approximation to the Ward Tordai equation, the adsorption parameters and surfactant diffusivities were obtained. This information, with an estimate of the drop deformation timescale, allowed estimation of the surface dilational modulus (Esd). This is a measure of the Marangoni stresses acting on the drop' surface due to interfacial tension gradients. Trends observed in the mean drop size and DSD experimental results are explained in terms of the interfacial and rheological properties. Below the CMC, Esd peaks and the drop size increases with concentration, despite a decrease in equilibrium interfacial tension. Above the CMC, Marangoni stresses are small but the presence of the surfactant still modifies the rheology of the interface, increasing the effective viscosity of the drops. A comprehensive set of mechanistic models for drop size in turbulent flows was developed and modified to partially account for the effect of surfactants via an appropriately defined effective viscosity. Various model choices were systematically fit to the drop size data to select the most appropriate mechanistic correlation. Normalized experimental DSD data collapsed to a single log-normal volume distribution.