Liquid uptake by fibrous absorbent materials

The thesis elucidates the mechanisms for fluid-material interactions within incontinence pads and helps to address the lack of understanding in this area. By providing a better understanding of liquid uptake by fibrous materials, the work will contribute to the design of better products for incontinence sufferers. A review of flow in porous media is presented covering the methods used to measure and model fluid transport, particularly relating to flow in textiles and other fibrous materials. The majority of the work described in the thesis focuses on the wicking properties of needle-felt fabrics. Such felts are used in reusable (washable) incontinence pads and also provide a simplified model for the more complex materials used in disposable products. An apparatus was designed, built, and used to measure wicking in textile materials. The simplified case of one-dimensional semi-infinite liquid uptake from an infinite reservoir was studied in detail for a range of felts. Mass uptake and wetted area were measured using a digital balance and camera. In particular, the impact on wicking of liquid temperature, sample orientation (horizontal, vertical and angles between), and felt compression were investigated. Wicking into textile materials is commonly understood using a simple capillary tube model for flow. To evaluate the application of capillary models the felt microstructure was examined. Encapsulated cross-sections of felt samples were prepared, and software written to identify fibres penetrating the plane of an examined section. While some aspects of liquid wicking, were found to be as expected from a simple capillary tube model, wicking appeared to be reduced in very open structured fibrous materials. It is suggested that this was due to the existence of unsaturated flow where only saturated flow was assumed. Considerable variation in liquid saturation was found in the samples during wicking

Superphobicity/philicity janus fabrics with Switchable, spontaneous, directional transport ability to water and oil fluids

Herein we demonstrate that switchable, spontaneous, directional-transport ability to both water and oil fluids can be created on fabric materials through wet-chemistry coating and successive UV irradiation treatment. When the fabric showed directional transport to a liquid, it prevented liquids of higher surface tension from penetration, but allowed liquids of lower surface tension to permeate, from either side. The directional transport ability can be switched from one fluid to another simply by heating the fabric at an elevated temperature and then re-irradiating the fabric with UV light for required period of time. By attaching liquid drops vertically upwards to a horizontally-laid fabric, we further demonstrated that this novel directional fluid transport was an automatic process driven by surface property alone, irrespective of gravity's effect. This novel fabric may be useful for development of “smart” textiles and functional membranes for various applications

A Review of Permeability and Flow Simulation for Liquid Composite Moulding of Plant Fibre Composites

Liquid composite moulding (LCM) of plant fibre composites has gained much attention for the development of structural biobased composites. To produce quality composites, better understanding of the resin impregnation process and flow behaviour in plant fibre reinforcements is vital. By reviewing the literature, we aim to identify key plant fibre reinforcement-specific factors that influence, if not govern, the mould filling stage during LCM of plant fibre composites. In particular, the differences in structure (physical and biochemical) for plant and synthetic fibres, their semi-products (i.e., yarns and rovings), and their mats and textiles are shown to have a perceptible effect on their compaction, in-plane permeability, and processing via LCM. In addition to examining the effects of dual-scale flow, resin absorption, (subsequent) fibre swelling, capillarity, and time-dependent saturated and unsaturated permeability that are specific to plant fibre reinforcements, we also review the various models utilised to predict and simulate resin impregnation during LCM of plant fibre composites

Wettability Switching Techniques on Superhydrophobic Surfaces

The wetting properties of superhydrophobic surfaces have generated worldwide research interest. A water drop on these surfaces forms a nearly perfect spherical pearl. Superhydrophobic materials hold considerable promise for potential applications ranging from self cleaning surfaces, completely water impermeable textiles to low cost energy displacement of liquids in lab-on-chip devices. However, the dynamic modification of the liquid droplets behavior and in particular of their wetting properties on these surfaces is still a challenging issue. In this review, after a brief overview on superhydrophobic states definition, the techniques leading to the modification of wettability behavior on superhydrophobic surfaces under specific conditions: optical, magnetic, mechanical, chemical, thermal are discussed. Finally, a focus on electrowetting is made from historical phenomenon pointed out some decades ago on classical planar hydrophobic surfaces to recent breakthrough obtained on superhydrophobic surfaces

Tuning Surface Wettability Through Volumetric Engineering

abstract: Many defense, healthcare, and energy applications can benefit from the development of surfaces that easily shed droplets of liquids of interest. Desired wetting properties are typically achieved via altering the surface chemistry or topography or both through surface engineering. Despite many recent advancements, materials modified only on their exterior are still prone to physical degradation and lack durability. In contrast to surface engineering, this thesis focuses on altering the bulk composition and the interior of a material to tune how an exterior surface would interact with liquids. Fundamental and applied aspects of engineering of two material systems with low contact angle hysteresis (i.e. ability to easily shed droplets) are explained. First, water-shedding metal matrix hydrophobic nanoparticle composites with high thermal conductivity for steam condensation rate enhancement are discussed. Despite having static contact angle <90° (not hydrophobic), sustained dropwise steam condensation can be achieved at the exterior surface of the composite due to low contact angle hysteresis (CAH). In order to explain this observation, the effect of varying the length scale of surface wetting heterogeneity over three orders of magnitude on the value of CAH was experimentally investigated. This study revealed that the CAH value is primarily governed by the pinning length which in turn depends on the length scale of wetting heterogeneity. Modifying the heterogeneity size ultimately leads to near isotropic wettability for surfaces with highly anisotropic nanoscale chemical heterogeneities. Next, development of lubricant-swollen polymeric omniphobic protective gear for defense and healthcare applications is described. Specifically, it is shown that the robust and durable protective gear can be made from polymeric material fully saturated with lubricant that can shed all liquids irrespective of their surface tensions even after multiple contact incidences with the foreign objects. Further, a couple of schemes are proposed to improve the rate of lubrication and replenishment of lubricant as well as reduce the total amount of lubricant required in making the polymeric protective gear omniphobic. Overall, this research aims to understand the underlying physics of dynamic surface-liquid interaction and provides simple scalable route to fabricate better materials for condensers and omniphobic protective gear.Dissertation/ThesisDoctoral Dissertation Mechanical Engineering 201

TRANSPORT PHENOMENA ASSOCIATED WITH LIQUID METAL FLOW OVER TOPOGRAPHICALLY MODIFIED SURFACES

Brazing and soldering, as advanced manufacturing processes, are of significant importance to industrial applications. It is widely accepted that joining by brazing or soldering is possible if a liquid metal wets the solids to be joined. Wetting, hence spreading and capillary action of liquid metal (often called filler) is of significant importance. Good wetting is required to distribute liquid metal over/between the substrate materials for a successful bonding. Topographically altered surfaces have been used to exploit novel wetting phenomena and associated capillary actions, such as imbibitions (a penetration of a liquid front over/through a rough, patterned surface). Modification of surface roughness may be considered as a venue to tune and control the spreading behavior of the liquids. Modeling of spreading of liquids on rough surface, in particular liquid metals is to a large extent unexplored and constitutes a cutting edge research topic. In this dissertation the imbibitions of liquid metal has been considered as pertained to the metal bonding processes involving brazing and soldering fillers. First, a detailed review of fundamentals and the recent progress in studies of non-reactive and reactive wetting/capillary phenomena has been provided. An imbibition phenomenon has been experimentally achieved for organic liquids and molten metals during spreading over topographically modified intermetallic surfaces. It is demonstrated that the kinetics of such an imbibition over rough surfaces follows the Washburn-type law during the main spreading stage. The Washburn-type theoretical modeling framework has been established for both isotropic and anisotropic non-reactive imbibition of liquid systems over rough surfaces. The rough surface domain is considered as a porous-like medium and the associated surface topographical features have been characterized either theoretically or experimentally through corresponding permeability, porosity and tortuosity. Phenomenological records and empirical data have been utilized to verify the constructed model. The agreement between predictions and empirical evidence appears to be good. Moreover, a reactive wetting in a high temperature brazing process has been studied for both polished and rough surfaces. A linear relation between the propagating triple line and the time has been established, with spreading dominated by a strong chemical reaction

Understanding liquid movements in textiles for the development of liquid repellent strategies

The understanding of the liquid movements in textiles is important to the development of novel liquid repellent strategies based on the manipulation of liquid motion. In this thesis we focus on the two areas that have received little attention: (1) the liquid permeation across the thickness of a single-layer textile following the deposition of a static droplet and (2) the liquid movements following the impact of droplets on a single-layer textile. In the study of area (1) we report a time-resolved high resolution X-ray imaging of the motion of the liquid-vapour interface in the textile thickness direction. The imaging of the time-dependent position of the interface is made possible by the use of ultra-high viscosity liquids (dynamic viscosity 2.5·106 times higher than that of water). Imaging results suggested a three-stage permeation mechanism with each stage being associated with one type of capillary channels in the textile geometry. We also showed that the permeation dynamics cannot be described by the popular Washburn theory. In the study of area (2) we record the impact of droplets on textiles with high-speed imaging. We showed that the impact on textiles at short timescales involved no droplet shape deformation if the textile’s porosity was sufficiently high. We also showed that droplets could be captured by the textiles under some impact conditions. By balancing the dynamic and capillary pressures we showed that the droplet penetration was governed by a threshold pore size and the droplet diameter. Moreover, we identified 5 stages for the liquid spreading on the textile surfaces following the impact. Within the investigated range of impact velocity the surface chemistry of the textiles was unimportant in the determination of liquid repellency. We also investigated the transplanar liquid permeation across non-wettable textiles following the deposition of droplets. We showed that the permeation was governed by a critical pore size and the weight of the deposited liquid. We discussed the limitation of the Gillespie scaling, developed for the prediction of in-plane spreading area in papers, in the description of the in-plane capillary spreading dynamics in textiles

ELASTO-CAPILLARITY IN FIBROUS MATERIALS

Current advances in the manufacture of nanoporous and nanofibrous materials with high absorption capacity open up new opportunities for the development of fiber-based probes and sensors. Pore structures of these materials can be designed to provide high suction pressure and fast wicking. During wicking, due to the strong capillary action, the liquids exert stresses on the fiber network, thus the stressed state of dry and wet parts of the material differs. In this work the effect of stress reduction in fibrous materials due to the presence of wetting liquid in the pore structure is studied in details for both static and dynamic cases. It is suggested that this effect can be used for liquid monitoring and the examples of one and two dimensional probes are provided. To open a discussion an illustrative example of a single capillary is considered and the effect of a moving meniscus on the stress distribution along capillary walls is demonstrated. Then the similar effects are analyzed in yarns and fabrics. A yarn that can capture an aerosol droplet is considered as a promising sensing element that could monitor the stresses caused by wetting fronts. It is shown that the stress transfer between dry and wet parts of the yarn upon liquid wicking significantly depends on the boundary conditions. The stress distribution in the yarn with clamped ends is discussed. The elasto-capillary problem is resolved for 2D case of a freely suspended self-reconfigurable material. It is shown that the classical Bernoulli problem of a freely suspended fabric can be used for the analysis of stresses in the fibrous matrix. The theoretical conclusions on elasto-capillarity are supported by experimental results on tensile testing of fibrous materials. The results show that the elasto-capillary effect is pronounced in the porous samples with the pore sizes smaller than 10 microns

Effect of fabric structure on liquid transport, ink jet drop spreading and printing quality

The effect of fabric structure and yarn-to-yarn liquid migration on the overall liquid transport behavior of fabrics is investigated in this research. Sorption of liquid from an unlimited reservoir as well as sorption of a limited quantity of liquid by fabrics representing different structural parameters is studied in detail. Sorption of a limited quantity of liquid is studied by performing drop spreading experiments on fabrics. The spreading and wicking of micron sized drops which are deposited on textile fabrics during ink jet printing is also studied. How the fabric structure related variables influence the spreading of ink drops and how exactly spreading influences printing quality is investigated in this research. Results showed that the wicking in fabrics is determined by the wicking rates of the yarns, thread spacing and more importantly by the rate at which liquid migrates from longitudinal to transverse threads and again from transverse threads back to longitudinal threads. Drop spreading rates were also determined by fabric structure. In general, compact and thinner fabrics showed highest drop spreading rates. Drop spreading rates are primarily affected by the manner and the rate at which liquid migrates from yarn to yarn. Analysis of the results of ink jet printing of pigment ink on textile fabrics showed that excessive drop spreading and higher line widths were observed where continuous and narrow capillaries prevail on the surface of yarns. Yarn surface characteristics are more important than fabric construction parameters.Ph.D.Committee Chair: Dr. Radhakrishnaiah Parachuru; Committee Member: Dr. Dong Yao; Committee Member: Dr. Fred Cook; Committee Member: Dr. Wallace Carr; Committee Member: Dr. Yehia El Mogahz