ABSTRACT:The production and control of jets, drops, bubbles and fluidic structures on the micro and nano-scale is of great interest in diverse technological fields including pharmacy, textile engineering or biomedicine. Regardless the strategy followed to produce the mentioned fluidic entities, in most applications one has to deal with some elements which considerably complicate the analysis, what we call complexity. In this dissertation we analyze the effects of the three sources of complexity usually found in microfluidics: electrical fields, viscoelasticity and addition of surfactants. Regarding the former, we focus on the minimum flow rate stability of electrospray. This technique makes use of intense electrical fields to produce very thin jets that break into droplets (paper I). To remove the electrical conditions imposed by electrospray, one can utilize the flow focusing principle, where similar drops/jets sizes are achieved by purely hydrodynamic means. In paper II we develop the viscoelastic gaseous flow focusing (VGFF) principle. Paper III and IV apply VGFF to smooth printing of microfilms and spinning of micro and nanofibers, respectively. To close the viscoelastic block, paper V assesses the validity of the Jeffreys model, traditionally used in the viscoelastic problems, to describe the oscillations of a viscoelastic pendant drop. Finally, during papers VI, VII and VIII we study the effects of the addition of surfactants in paradigmatic problems of surface tension driven flows, namely, oscillation and breakup of capillary systems. Special attention is paid to the role played by soluto-capillarity, Marangoni convection and surface viscosities.RESUMEN:La producción y control de chorros, gotas, burbujas y otras estructuras fluídicas en la escala micro y nanométrica es de enorme interés en campos tan variados como farmacia, ingeniería textil o biomedicina. Independientemente del método de producción empleado, la mayoría de aplicaciones involucran algunos elementos que complican considerablemente el análisis, lo cual llamamos complejidad. La presente tesis analiza los efectos de las tres fuentes de complejidad más frecuentes en microfluídica: campos eléctricos, viscoelasticidad y adición de surfactantes. Respecto a la primera de ellas, esta tesis se centra en el límite de estabilidad de caudal mínimo de electrospray. Esta técnica produce, mediante intensos campos eléctricos, chorros muy finos que rompen en gotas (artículo I). Flow focusing permite producir, mediante medios puramente hidrodinámicos, chorros y gotas con tamaños similares a electrospray. El artículo II desarrolla la versión viscoelástica del flow focusing gaseoso (VGFF). Los papers III y IV aplican el VGFF a la impresión de películas micrométricas y la fabricación de fibras micro y nanométricas, respectivamente. Para cerrar el bloque viscoelástico, el paper V estudia la validez del modelo de Jeffreys, ampliamente empleado en problemas viscoelásticos, para describir la oscilación de gotas pendientes viscoelásticas. Por último, los artículos VI, VII y VIII estudian el efecto de la adición de surfactantes en problemas paradigmáticos de flujos dominados por la tensión superficial como son la oscilación y la rotura de sistemas capilares. Se presta especial atención al papel que juegan la solutocapilaridad, la convección de Marangoni y las viscosidades superficiales.BIBLIOGRAPHY/BIBLIOGRAFÍAAmbravaneswaran, B. and Basaran, O. A. (1999). Effects of insoluble surfactants on the nonlinear deformation and breakup of stretching liquid bridges. Phys. Fluids, 11:997-1015.Anna, S. L. (2016). Droplets and bubbles in microfluidic devices. Annu. Rev. Fluid Mech., 48:285-309.Anna, S. L., Bontoux, N., and Stone, H. A. (2003). Formation of dispersions using flow focusing in microchannels.Appl. Phys. Lett., 82:364-366.Basaran, O. A. (2002). Small-scale free surface flows with breakup: Drop formation and emerging applications.AlChE J., 48:1842-1848.Basaran, O. A., Gao, H., and Bhat, P. P. (2013). Nonstandard inkjets. Annu. Rev. Fluid Mech., 45:85-113.Bazilevskii, A. V., Entov, V. M., and Rozhkov, A. N. (2001). Breakup of an oldroyd liquid bridge as a method for testing the rheological properties of polymer solutions. Polym. Sci. Ser. A, 42:716-726.Becker, E., Hiller, W. J., and Kowalewski, T. A. (1991). Experimental and theoretical investigation of large-amplitude oscillations of liquid droplets. J. Fluid Mech., 231:189-210.Bird, R. B., Armstrong, R. C., and Hassager, O. (1987). Dynamics of Polymeric Liquids. John Wiley & Sons, Inc., United States of America.Castrejón-Pita, J. R., Castrejón-Pita, A. A., Thete, S. S., Sambath, K., Hutchings, I. M., Hinch, J., Lister,J. R., and Basaran, O. A. (2015). Plethora of transitions during breakup of liquid laments. Proc. Natl. Acad. Sci., 112:4582-4587.Chapman et al., H. N. (2011). Femtosecond x-ray protein nanocrystallography. Nature, 470:73-79.Cloupeau, M. and Prunet-Foch, B. (1989). Electrostatic spraying of liquids in cone-jet mode. J. Electrostatics, 22:135-159.Cohen, I., Li, H., Hougland, J. L., Mrksich, M., and Nagel, S. R. (2001). Using selective withdrawal to coat microparticles. Science, 292:265-267.Conroy, D. T., Matar, K., Craster, R. V., and Papageorgiou, D. T. (2011). Breakup of an electried viscousthread with charged surfactants. Phys. Fluids, 23:022103.Craster, R. V., Matar, O. K., and Papageorgiou, D. T. (2002). Pinchoff and satellite formation in surfactant covered viscous threads. Phys. Fluids, 14:1364-1376.Craster, R. V., Matar, O. K., and Papageorgiou, D. T. (2009). Breakup of surfactant-laden jets above the critical micelle concentration. J. Fluid Mech., 629:195-219.Denn, M. M. (1980). Continuous drawing of liquids to form bers. Ann. Rev. Fluid Mech., 12:365{387. DePonte, D. P., Weierstall, U., Schmidt, K., Warner, J., Starodub, D., Spence, J. C. H., and Doak, R. B. (2008). Gas dynamic virtual nozzle for generation of microscopic droplet streams. J. Phys. D: Appl. Phys., 41:195505.Derzsi, L., Kasprzyk, M., Plog, J. P., and Garstecki, P. (2013). Flow focusing with viscoelastic liquids. Phys. Fluids, 25:092001.Doshi, J. and Reneker, D. R. (1995). Electrospinning process and applications of electrospun fibers. J.Electrost., 35:151-60.Dravid, V., Songsermpong, S., Xue, Z., Corvalan, C. M., and Sojka, P. E. (2006). Two-dimensional modeling of the effects of insoluble surfactant on the breakup of a liquid filament. Chem. Eng. Sci, 61:3577-3585.Duboin, A., Middleton, R., Malloggi, F., Monti, F., and Tabeling, P. (2013). Cusps, spouts and microfiber synthesis with microfluidics. Soft Matter, 9:3041-3049.Eggers, J. (1993). Universal pinching of 3D axisymmetric free-surface flow. Phys. Rev. Lett., 71:3458-3460.Eggers, J. and Villermaux, E. (2008). Physics of liquid jets. Rep. Prog. Phys., 71:036601.Elfring, G. J., Leal, L. G., and Squires, T. M. (2016). Surface viscosity and Marangoni stresses at surfactantladen interfaces. J. Fluid Mech., 792:712-739.Fenn, J. B., Mann, M., Meng, C. K., Wong, S. F., and Whitehouse, C. M. (1989). Electrospray ionizationfor mass spectrometry of large biomolecules. Science, 246:64-71.Galindo-Rosales, F. J., Alves, M. A., and Oliveira, M. S. N. (2013). Microdevices for extensional rheometryof low viscosity elastic liquids: a review. Microfluid Nanofluid, 14:1-19.Gamero-Castaño, M. and Hruby, V. (2001). Electrospray as a source of nanoparticles for effcient colloid thrusters. J. Propul. Power., 17:977-987.Gañán-Calvo, A. M. (1998). Generation of steady liquid microthreads and micron-sized monodisperse spraysin gas streams. Phys. Rev. Lett., 80:285-288.Gañán-Calvo, A. M., López-Herrera, J. M., Herrada, M. A., Ramos, A., and Montanero, J. M. (2018). Review on the physics electrospray: from electrokinetics to the operating conditions of single and coaxial Taylor cone-jets, and AC electrospray. J. Aerosol Sci., 125:32-56.Gañán-Calvo, A. M., Montanero, J. M., Martín-Banderas, L., and Flores-Mosquera, M. (2013). Building functional materials for health care and pharmacy from microfluidic principles and Flow Focusing. Adv. Drug Delivery Rev., 65:1447-1469.Gañán-Calvo, A. M., Rebollo-Muñoz, N., and Montanero, J. M. (2013). Physical symmetries and scaling laws for the minimum or natural rate of flow and droplet size ejected by Taylor cone-jets. New J. Phys., 15:033035.Herrada, M. A., Montanero, J. M., and Vega, J. M. (2011). The effect of surface shear viscosity on thedamping of oscillations in millimetric liquid bridges. Phys. Fluids, 23:082102.Jaworek, A. (2007). Electrospray droplet sources for thin film deposition. J. Mater. Sci., 42:266-297.Kamat, P. M., Wagoner, B. W., Thete, S. S., and Basaran, O. A. (2018). Role of Marangoni stress during breakup of surfactant-covered liquid threads: Reduced rates of thinning and microthread cascades. Phys. Rev. Fluids, 3:043602.Kovalchuk, N. M., Nowak, E., and Simmons, M. J. H. (2016). Effect of soluble surfactants on the kinetics of thinning of liquid bridges during drops formation and on size of satellite droplets. Langmuir, 32:5069-5077.Lee, W., Walker, L. M., and Anna, S. L. (2011). Competition between viscoelasticity and surfactant dynamics in flow focusing microfluidics. Macromol. Mater. Eng., 296:203-213.Liao, Y.-C., Franses, E. I., and Basaran, O. A. (2006). Deformation and breakup of a stretching liquid bridge covered with an insoluble surfactant monolayer. Phys. Fluids, 18:022101.Liao, Y.-C., Subramani, H. J., Franses, E. I., and Basaran, O. A. (2004). Effects of soluble surfactants on the deformation and breakup of stretching liquid bridges. Langmuir, 20:9926-9930.Madurga, R., Guinea, G. V., Elices, M., Pérez-Rigueiro, J., and Gañán-Calvo, A. M. (2017). Straining flow spinning: Simplied model of a bioinspired process to mass produce regenerated silk bers controllably. Eur. Polymer J., 97:26-39.Mayer, H. C. and Krechetnikov, R. (2012). Landau-levich flow visualization: Revealing the flow topology responsible for the film thickening phenomena. Phys. Fluids, 24:052103.McGough, P. T. and Basaran, O. A. (2006). Repeated formation of fluid threads in breakup of a surfactant coveredjet. Phys. Rev. Lett., 96:054502.McKinley, G. H. (2005). Visco-elasto-capillary thinning and break-up of complex fluids. Rheology Reviews, 2005:1-49.McKinley, G. H. and Sridhar, T. (2002). Filament-stretching rheometry of complex fluids. Annu. Rev. Fluid Mech., 34:375-415.Miller, E., Rotea, M., and Rothstein, J. P. (2010). Microfluidic device incorporating closed loop feedback control for uniform and tunable production of micro-droplets. Lab on a Chip, 10:1293-1301.Morrison, F. A. (2001). Understanding Rheology. Oxford University Press, Oxford, UK.Oldroyd, J. G. (1950). On the formulation of rheological equations of state. Proc. Roy. Soc. Lond., 200:523-541.Oliveira, M. S. N., Pinho, F. T., Poole, R. J., Oliveira, P. J., and Alves, M. A. (2009). Purely elastic flow asymmetries in flow-focusing devices. J. Non-Newtonian Fluid Mech., 160:31-39.Orme, M., Liu, Q., and Smith, R. (2000). Molten aluminum micro-droplet formation and deposition for advanced manufacturing applications. Aluminum Transactions, 3:95-103.Prosser, A. J. and Franses, E. I. (2001). Adsorption and surface tension of ionic surfactants at the air-water interface: review and evaluation of equilibrium models. Colloids and Surfaces A, 178:1-40.Roche, M., Aytouna, M., Bonn, D., and Kellay, H. (2009). Effect of surface tension variations on the pinch-off behavior of small fluid drops in the presence of surfactants. Phys. Rev. Lett., 103:264501.Scriven, L. E. (1960). Dynamics of a fluid interface. Equation of motion for Newtonian surface fluids. Chem. Eng. Sci., 12:98-108.Scriven, L. E. and Sternling, C. V. (1960). The Marangoni effects. Nature, 187:186-188.Steinhaus, B., Shena, A. Q., and Sureshkumar, R. (2007). Dynamics of viscoelastic fluid filaments in microfluidic devices. Phys. Fluids, 19:073103.Stevenson, P. (2005). Remarks on the shear viscosity of surfaces stabilised with soluble surfactants. J. Colloid Interface Sci., 290:603-606.Stone, H. A. and Leal, L. G. (1990). The effects of surfactants on drop deformation and breakup. J . Fluid Mech., 220:161-186.Sun, D., Chang, C., Li, S., and Lin, L. (2006). Near-field electrospinning. Nano Lett., 6:839-842.Suryo, R. and Basaran, O. A. (2006). Tip streaming from a liquid drop forming from a tube in a coflowingouter fluid. Phys. Fluids, 18:082102.Taylor, G. (1964). Disintegration of water drops in electric field. Proc. R. Soc. Lond. A, 280:383-397.Theolis, V. (2011). Global linear instability. Annu. Rev. Fluid Mech., 43:319-352.Timmermans, M.-L. E. and Lister, J. R. (2002). The effect of surfactant on the stability of a liquid thread. J. Fluid Mech., 459:289-306.Xie, J., Jiang, J., Davoodi, P., Srinivasan, M. P., andWang, C. (2015). Electrohydrodynamic atomization: A two-decade effort to produce and process micro-/nanoparticulate materials. Chem. Eng. Sci., 125:32-57.Zell, Z. A., Nowbahar, A., Mansard, V., Leal, L. G., Deshmukh, S. S., Mecca, J. M., Tucker, C. J., and Squires, T. M. (2014). Surface shear inviscidity of soluble surfactants. Proc. Natl. Acad. Sci., 111:3677-3682.Zhou, C., Yue, P., and Feng, J. J. (2006). Formation of simple and compound drops in microfluidic devices. Phys. Fluids, 18:092105.