Study of the physicochemical properties of micro- and nano-dimensional systems
Creators: name: Kaluarachchi, Chathuri Piumika
Publisher: name: University of Iowa
Year: 2022
Subjects: public abstract

description: Characterization of the physicochemical (i.e., morphology, composition, phase state, water uptake, surface tension), and mechanical (i.e., stiffness) properties of micro- and nanodimensional environmental samples (i.e., marine aerosols and air-liquid interfaces), and biological systems (i.e., live biological cells) under ambient conditions have been challenging in part due to their complexity and intrinsic size limitations. However, characterizing these properties is highly beneficial to better understand their behavior in the atmosphere and within the human body, respectively. Thus, an introduction of novel scanning probe techniques such as atomic force microscopy (AFM) and development of accompanied methods of measurement and analysis have led to advancements in this regard. Herein, this dissertation focuses on AFM-based methods development and application towards the investigation and characterization of physicochemical properties and mechanical properties of various environmentally and biologically relevant systems at both micro- and nano-scales under ambient conditions.
description: The AFM-based methodology was developed to quantify the organic volume fraction (OVF, the relative organic shell volume with respect to the total particle volume) of individual core-shell particles using glucose and NaCl binary mixtures with different molar ratios as model systems. Findings illustrate an accuracy of this approach on quantifying the single particle OVF, which was then extended on real individual core-shell sea spray aerosol (SSA) particles. Furthermore, results illustrated an importance of recognizing how conventional particle deposition methods based on impaction on solid surfaces may significantly alter the impacted particle shape and size.
description: The impact of SSA on Earth’s climate remains uncertain in part due to size-dependent particle-to-particle variability in their physicochemical properties. Thus, AFM-based v characterization of these physicochemical properties for substrate deposited individual submicrometer nascent SSA (nSSA) and atmospherically aged SSA were performed. Results showed a significant variability in morphology, composition, water uptake, and phase state as a function of particle size, relative humidity (RH), and biological activity in seawater. Moreover, findings showed that atmospheric aging of nSSA can further alter their physicochemical properties (e.g., change in water uptake efficiency). In addition, we revealed a dynamic and inter-related nature of these physicochemical properties, emphasizing the need for accurate single particle level studies to better understand the effect of SSA on the Earth’s climate and atmosphere.
description: AFM ability to quantify the surface tension of aqueous liquid-air interfaces with (sub)micrometer indentation depths was investigated using solution droplets of binary mixture model solutions (hexanoic acid and water) as a function of hexanoic acid aqueous solution concentration. We showed the accuracy and significance of AFM-based method to quantify the surface tension of micron scale liquid-air interfaces, which otherwise cannot be studied with more traditional surface tension measurement techniques, such as tensiometer as it requires significantly larger probing distances into the liquid-air interface. AFM-based methodology was also utilized to reveal small, yet important, differences in the surface tension of complex air-liquid interfaces such as sea surface microlayer (SML), where the type and concentration of surfactants is expected to vary with distance away from the air-liquid interface.
description: AFM capability to quantify the stiffness of soft biological cells under liquid environment was investigated using Drosophila follicles. A key regulator of collective cell migration, which drives development and cancer metastasis, is the substrate stiffness. Increased substrate stiffness promotes migration, and the substrate stiffness is controlled by Myosin. We identified that Fascin can regulate Myosin activity. Our findings revealed that the loss of Fascin results in increased vi activated Myosin on the border cells and their substrate, and increased nurse cell stiffness as measured by AFM. Thus, we showed that migrating border cells play a critical role in controlling the mechanical properties of their substrate to promote their migration during the cancer metastasis.