Fouling Physics on Soft Materials and the Rational Engineering of Antifouling Heat Transfer Surfaces

Abstract

Fresh water and energy are essential resources in our daily lives, interdependent, and vital for numerous processes. Water is necessary for energy production as a cooling medium, while energy is needed for effective water treatment and distribution. The limited availability of these resources, combined with challenges like climate change and population growth, stresses their interconnection, known as the water-energy nexus. A key challenge within this nexus is fouling or scaling, which involves the buildup of unwanted deposits from microscale particles and inverse soluble minerals like calcium carbonate and calcium sulfate. Fouling impairs water treatment and energy production, causing energy losses and high costs. Despite previous efforts to develop materials and strategies to prevent and reduce fouling, the problem persists. This is partly due to an incomplete understanding of the underlying mechanisms of fouling, which are crucial for guiding the design of antifouling and scalephobic materials to improve efficiency and reduce stress within the water-energy nexus. In this thesis, we aim to counteract this by exploring the complex phenomena of fouling and establishing a scientific base for its fundamental physics. We develop in situ measurement techniques to analyze and gain a fundamental understanding of microscale fouling formation and removal, leading to effective design guidelines for creating antifouling and scalephobic materials to prevent and minimize fouling. In the first study, we introduce an advanced in situ measurement methodology that integrates fluidic and adhesion theories to examine microfoulant adhesion and removal on compliant engineered surfaces. We analyze the effects of interfacial hydrodynamics, material compliance, wettability, and surface microtexture on removing microfoulants such as calcium carbonate and microplastic particles. Unlike previous ex situ studies, we find that altering the wettability of rigid materials does not impact removal performance, highlighting the importance of in situ studies. We identify three primary microfoulant removal mechanisms: gliding, rolling, and shedding, and demonstrate that surface microtexture enhances removal efficiency through shedding. Our results suggest that antifouling materials should be tailored to the specific fouling mechanism: rigid coatings are effective for particulate fouling, while soft coatings are better for crystallization fouling. However, excessive compliance can hinder removal due to indentation and lubrication-induced gliding. Based on these insights, we design and successfully test a microtextured compliant superhydrophilic hydrogel coating with superior scale-shedding properties under various flow conditions. In outlook experiments, we explore combining the designed surfaces with noninvasive removal techniques, such as external acoustic fields, showing promising preliminary results. The second study investigates in situ the deposition of calcium sulfate scale on engineered metallic heat transfer surfaces. We develop a continuous flow fouling unit with microscopic resolution and heat transfer quantification to examine the effects of material composition and surface structure on scale formation and its impact on heat transfer resistance under hydrodynamic shear flow. Our in situ optical characterization and thermofluidic modeling reveal that degassing-induced bubble formation creates local hot spots and enhances supersaturation, substantially impacting scaling and heat transfer. We quantify bubble appearance, residence time, and coverage based on surface composition and structure. These findings suggest that suppressing surface bubbles is essential to prevent fouling. We, therefore, designed a nanostructured superhydrophilic scalephobic surface that prevents bubble formation, reduces scale deposition, and maintains high heat transfer efficiency. In the third study, we leverage the findings from the first two studies to evaluate the scalephobic properties of a hydrogel under severe crystallization fouling conditions in a liquid cooling flow unit. Our initial findings indicate that the soft hydrogel demonstrates substantially prolonged experimental duration compared to a rigid substrate before dense intergrown crystal clusters develop. We attribute this delay to the detachment of individual crystals and crystal clusters, a phenomenon not previously observed in situ under accelerated severe fouling conditions. The durability and enhanced cooling efficiency of the scalephobic hydrogel demonstrate its potential to address fouling to reduce stress on water and energy resources, paving the way for a sustainable future.