Dynamics of Colloids Interacting with the Liquid-Liquid Interfaces of Aqueous Two-Phase Systems

Abstract

Active colloids, such as motile bacteria, not only interact with complex environments in nature, but they move to survive such as finding food and oxygen, avoiding toxic substances, and navigating to better places to live. The aqueous complex environment, such as chemotaxis, aerotaxis, wall effect, and polymeric solution determines the bacterial motion. When colloids are first dispersed in an aqueous solution, a water-bacterium interface forms, where interfacial tension dictates the behavior of passive colloids. Especially in a multi-aqueous system where an interface separates different water-based solu- tions, the interfacial governing behavior is the major dominant contribution. However, the interaction between the active colloids and the interfacial tension remains unknown. This dissertation contributes insight into the physics of the interaction between passive/active colloids and the ultra-low interfacial tension of liquid-liquid interface in a quasi-2D environment. Chapter 2 first focuses on the passive colloid, polystyrene functionalized by the carboxyl group (PS- COOH), dispersed in the aqueous two-phase system (ATPS) consisting of dextran (DEX) and polyethy- lene glycol (PEG) which is derived into DEX-rich and PEG-rich phases. All PS-COOH are partitioned into DEX-rich phase because it is energetically favorable. And the partitioning of PS-COOH obeys the Boltzmann statistics, CPEG−rich CDEX−rich = exp(−(EPEG−rich −EDEX−rich)/kBT ) = exp(−∆E/kBT ). Where E de- scribes the interaction energy of the colloids in the mixture and ∆E = E2 −E1 is the interaction energy difference of colloid between two phases. By crossing the interface with colloid, an optical tweezer measure reveals ∆E ≫ kBT that the Brownian colloids are all partitioned into DEX-rich phase. In Chapter 3, active colloids are dispersed in an ATPS where still ∆E ≫ kBT and lowest in DEX-rich, however the activity of colloid drive the system non-equilibrium that do not satisfy the Boltzman statis- tics. The overall partitioning is interaction between the colloidal preference to dextran and the activity. The model system of this chapter is non-motile or motile bacteria, Bacillus subtilis, living in ATPS. While all the non-motile bacteria reside in the DEX-rich phase like the passive colloids, motile bacteria are present in the DEX-rich and PEG-rich phases. Based on our measurements of DEX concentration- dependent partitioning ratios and the maximum required force to the crossing interface of the bacterium, we theoretically check the hypothesis that the bacteria in the DEX-rich phase are energetically favorable, but this soft confinement is overcome by active motility, to enter the PEG-rich phase. The theory predicts the partitioning ratio based on the competition between interfacial forces (strength of the confinement) and bacterial propulsion (strength of motility) where the dilute active rods are within a periodic soft confinement potential. Chapter 4 active colloid is dispersed in ATPS where ∆E ∼ 0 and interact with the liquid-liquid in- terface. The model system of ATPS is isotropic-nematic coexistence phase of an aqueous chromonic liquid crystal and swimming bacteria, B. subtilis, are dispered in there. Focusing on the bacteria trajec- tories near and at the liquid-liquid interfaces, the ultra-low interfacial tension of the isotropic-nematic interface justifies that bacteria swimming more perpendicular to the interface have a higher probability of crossing the interface. The force-balance model, considering the interfacial tension, further predicts how the length and speed of the bacteria affect their crossing behaviors. Finally, Chapter 5 studies the swimming of B. subtilis in isotropic and nematic phases and polymeric solutions. Observing the flagella conformation of B. subtilis, the flagellar bundle merges immediately upon entering the nematic phase from the isotropic phase, resulting in faster wiggling, a suppressed wiggling angle, and a shorter wavelength.