An atomic force microscope (AFM) has been used to conduct force measurements to better understand the role of hydrophobic forces in flotation. The force measurements were conducted between a flat mineral substrate and a hydrophobic glass sphere in aqueous solutions. It is assumed that the hydrophobic glass sphere may simulate the behavior of air bubbles during flotation. The results may provide information relevant to the bubble-particle interactions occurring during flotation. The glass sphere was hydrophobized by octadecyltrichlorosilane so that its water contact angle was 109 degrees. The mineral systems studied include covellite (CuS), sphalerite (ZnS) and hornblende (Ca₂(Mg, Fe)₅(Si₈O₂₂)(OH,F)₂). The collector used for all the mineral systems studied was potassium ethyl xanthate (KEX). For the covellite-xanthate system, a biopotentiostat was used in conjunction with the AFM to control the potential of the mineral surface during force measurements. This was necessary since the adsorption of xanthate is strongly dependent on the electrochemical potential (Eₕ) across the solid/liquid interface. The results show the presence of strong hydrophobic forces not accounted for by the DLVO (named after Derjaguin, Landau, Verwey and Overbeek) theory. Furthermore, the potential at which the strongest hydrophobic force was measured corresponds to the potential where the flotation recovery of covellite reaches a maximum, indicating a close relationship between the two. Direct force measurements were also conducted to study the mechanism of copper-activation of sphalerite. The force measurements conducted with unactivated sphalerite in 10⁻³ M KEX solutions did not show the presence of hydrophobic force while the results obtained with copper-activated sphalerite at pH 9.2 and 4.6 showed strong hydrophobic forces. However, at pH 6.8, no hydrophobic forces were observed, which explains why the flotation of sphalerite is depressed in the neutral pH regime. Direct force measurements were also conducted using hornblende in xanthate solutions to study the mechanism of inadvertent activation and flotation of rock minerals. The results show the presence of long-range hydrophobic forces when hornblende was activated by heavy metal cations such as Cu²⁺ and Ni²⁺ ions. The strong hydrophobic forces were observed at pHs above the precipitation pH of the activating cation. These results were confirmed by the XPS analysis of the activated hornblende samples. Force measurements were conducted between silanated silica surfaces to explore the relationship between hydrophobicity, advancing contact angle (CA), and the magnitude (K) of hydrophobic force. In general, K increases as Contact Angle increases and does so abruptly at Contact Angle=90°. At the same time, the acid-base component of the surface free energy decreases with increasing CA and K. At CA>90°, GammaS^AB approaches zero. Based on the results obtained in the present work a mathematical model for the origin of the hydrophobic force has been developed. It is based on the premise that hydrophobic force originates from the attraction between large dipoles on two opposing surfaces. The model has been used successfully to fit the measured hydrophobic forces using dipole moment as the only adjustable parameter. However, the hydrophobic forces measured at CA>90° cannot be fitted to the model, indicating that there may be an additional mechanism, possibly cavitation, contributing to the appearance of the long-range hydrophobic force. / Ph. D.

This thesis addresses development of a new force spectroscopy tool, correlation force spectroscopy (CFS), for the measurement of the mechanical properties of very small volumes of material (molecular to µm³) at kHz-MHz time-scales. CFS is based on atomic force microscopy (AFM) and the principles of CFS resemble those of dual-trap optical tweezers. CFS consists of two closely-spaced micro-cantilevers that undergo thermal fluctuations. Measurement of the correlation in thermal fluctuations of the two cantilevers can be used to determine the mechanical properties of the soft matter, e.g. a polymeric molecule, that connects the gap between the two cantilevers. Modeling of the correlations yields the effective stiffness and damping of the molecule. The resolution in stiffness is limited by the stiffness of the cantilever and the frequency by the natural frequency of the cantilevers, but, importantly, the damping resolution is not limited by the damping of the cantilever, which has enabled high-resolution measurements of the internal friction of a polymer. The concept of CFS was originally presented by Roukes' group in Caltech [Arlett et al., Lecture Notes in Physics, 2007]; I developed the first practical versions of CFS for experimentation, and have used it in two applications (1) microrheology of Newtonian fluids and (2) single molecule force spectroscopy. To understand the correlation in thermal fluctuations of two cantilevers I initially validated the theoretical approach for analyzing correlation in terms of deterministic model using the fluctuation-dissipation theorem [Paul and Cross, PRL, 2004]. I have shown that the main advantages of such correlation measurements are a large improvement in the ability to resolve stiffness and damping. Use of CFS as a rheometer was validated by comparison between experimental data and finite element modeling of the deterministic vibrations of the cantilevers using the known viscosity and density of fluids. Work in this thesis shows that the data can also be accurately fitted using a simple harmonic oscillator model, which can be used for rapid rheometric measurements, after calibration. The mechanical properties of biomolecules such as dextran and single stranded DNA (ssDNA) are also described. CFS measurements of single molecule properties of ssDNA reveal the internal friction of the molecule in solution. / Ph. D.

Cells in the body interact with a predominantly fibrous microenvironment and constantly adapt to changes in their neighboring physiochemical environment, which has implications in developmental and disease biology. A myriad of in vitro platforms including 2D flat and 3D gel substrates with and without anisotropy have demonstrated cellular alterations to subtle changes in topography. Recently, our work using suspended fibers as a new in vitro biological assay has revealed that cells are able to sense and respond to changes in fiber curvature and structural stiffness as evidenced by alterations to cytoskeleton arrangement, including focal adhesion cluster lengths and nucleus shape indices, leading to altered migration speeds. It is hypothesized that these behaviors occur due to modulation of cellular inside-out forces in response to changes in the external fibrous environment (outside-in). Thus, in this study, we investigate the role of fiber curvature and structural stiffness in force modulation of single cells attached to suspended fibers. Using our previously reported non-electrospinning Spinneret based Tunable Engineered Parameters (STEP) fiber manufacturing platform, we present our findings on single cell inside-out and outside-in forces using fibers of three diameters (250 nm, 400 nm and 800 nm) representing a wide range of structural stiffness (3-45 nN/μm). To investigate cellular adaptability to external perturbation, we present the development of a first-of-its-kind force measurement 'nanonet' platform capable of investigating cell adhesion forces in response to symmetric and non-symmetric (injury model) loading. Our combined findings are multi-fold: (i) Cells on suspended fibers are able to form focal adhesion clusters approximately four times longer than those on flat substrates, which gives them potential to double their migration speeds, (ii) Nanonets as force probes show that the contractility-based inside-out forces are nearly equally distributed on both sides of the cell body, and that overall force magnitudes are dependent on fiber structural stiffness, and (iii) External perturbation can evenly (symmetric) or unevenly (non-symmetric) distribute forces within the cell, and the resulting bias causes diameter-dependent outside-in adhesion force response. Finally, we demonstrate the power of the developed force measurement platform by extending our studies to cell-cell junctional forces as well as single-cell disease models including cancer and aortic aneurysm. / Ph. D.

Force feedback during teleoperation and in Virtual Reality (VR) environments is becoming increasingly common. We are interested in understanding the impact of motion on the directional accuracy of force perception, as observed in a VR environment. We used a custom force-feedback system that pulled a handle with a force of 1.87N at various angles in front of N=14 subjects. The virtual environment showed a curved wall, which corresponded to the locations from which the force could physically originate. Subjects selected where they perceived the force to originate from with a virtual laser pointer and by orienting their head. We compared several conditions: the subject held the handle still; the subject moved the handle back and forth toward the center of the wall; the subject moved the handle back and forth across their body; and the subject moved the handle back and forth toward where they thought the force was originating. Subjects were able to localize the force with an average accuracy of 1-10 degrees depending on the force's location, which is better than previous studies. All conditions had similiar accuracies. Subjects had the best precision when they followed the force as compared to either of the other conditions with movement. / Master of Science / In recent years, robots combined with teleoperation, operating in a remote safe environment, has become a popular choice for replacing human workers in dangerous environments. Visual feedback and a sense of touch and motion, are two of the most common feedback modalities. Thus, Virtual Reality (VR) and force rendering are two main ways of conveying information to the operator during teleoperation.\newline Previous studies have investigated the effects of force feedback on the fingers, wrist, and arms but with limited movements and joint combinations. In this paper, we answered the question of how the planar arm movement impacts the force-directional perception accuracy by using a Virtual Reality (VR) system. To put in other words, we want to find out how accurate and precise a robot operator can feel the physical world through joysticks. If they are asked to do this many times in a row, how repeatable are their guesses? To study this, We asked subjects holding a handle made out of PVC pipe with a position sensor on it. The handle was attached to a motor, which pulled the handle away from the subjects during the experiment trial. The experiment consisted of four different conditions, which studied both stationary, when subjects holding the handle stationary and resist the pull by our motor, and movement, when subjects moving the handle in a certain direction while the handle was pulled by our motor. In each trial, subjects were first asked to resist the force according to the experiment condition, then use a laser pointer and head to both point and look at where they think the motor was pulling the handle from. Because of the use of the VR environment and position sensor, subjects reported their guesses intuitively by pointing and looking at, which eliminated the potential of misreporting guesses. The result of this study is important for designing an effective force feedback system for teleoperation. With this information, a force feedback system in a VR environment could be altered to convey information to a user more accurately, for example to correct any biases that the user may have in where they expect forces to originate.\newline Our results show that arm movements enhanced the force feedback precision without sacrificing the accuracy. Arm movements also improved the subjects' confidence level in how well they thought they could localize a force. The results also suggest that pointing with the head is significantly more precise compared with the hand. Such results can be used to implement a more effective force feedback system combined with a VR environment. Finally, our data also shows that hand had an opposite accuracy pattern compared with the head. Future works are needed to explain this opposite accuracy pattern.

No / The ability of a cell to adhere and transmit traction forces to a surface reveals the cytoskeleton integrity of a cell. Shear sensitive liquid crystals were discovered with new function in sensing cell traction force recently. This liquid crystal has been previously shown to be non-toxic, linear viscoelastic and sensitive to localized exerted forces. This paper reports the possibility of extending the application of the proposed liquid crystal based cell force sensor in sensing traction forces of osteoblast-like (MG-63) and human keratinocyte (HaCaT) cell lines exerted to the liquid crystal sensor. Incorporated with cell force measurement software, force distributions of both cell types were represented in force maps. For these lowly contractile cells, chondrocytes expressed regular forces (10 – 90 nN, N = 200) around the circular cell body whereas HaCaT projected forces (0 – 200 nN, N = 200) around the perimeter of poly-hedral shaped body. These forces are associated with the organisation of the focal adhesion expressions and stiffness of the LC substrate. From the results, liquid crystal based cell force sensor system is shown to be feasible in detecting forces of both MG63 and HaCaT.

Eddy current separation aims to recover non-ferrous metals from non-metals utilizing electromagnetic interactions. In order to describe the separation process, a representative model is needed that can accurately calculate the induced forces. Such a model can be used to optimize the efficiency of current equipment as well as designing ones that can offer new capabilities. Models proposed so far for the separation process, using traditional approaches to calculate forces, had limited success due to complex nature of electromagnetic interactions. In this dissertation, a novel method for calculating the magnetic force acting on non-ferrous metal particles was developed. By this method, force calculations can be carried out accurately using intrinsic parameters of particles such as size and shape, as well as its orientation within the field. The method also takes into account the operating parameters of the equipment such as the rotational speed of the magnetic element and the speed of the belt. In order to verify this method and collect empirical data, a novel data acquisition and interpretation approach was developed. A computer simulator was also developed that can calculate trajectories of particles based on operating parameters of the eddy current separator and characteristics of the material being processed. The accuracy of the simulator was verified using empirical data obtained by the novel data acquisition method. This contribution provides a viable option for reducing the cost of analyzing; optimizing and designing eddy current separators. / PHD / As technological advances in chemistry, material science, engineering and manufacturing lead to building of items with smaller parts and complex components, recycling them is becoming more challenging. Production of raw materials, especially metals, from fresh ores in mining has become a challenge due to rising costs and depletion of high grade deposits. Thus, in order to sustain growth of the economy and advances in technology, recycling is of utmost importance. Iron and iron containing metal alloys such as steel can easily be separated with magnets. In order to separate metals that do not contain iron, such as aluminum, copper, brass, lead and zinc, eddy current separators are used. Until now, it was not possible to define the separation process as a whole fundamentally based on the characteristics of particles and operating parameters of eddy current separators. In this research, new methods to analyze the separation process as well as a new technique to calculate the magnetic force acting on metal particles were developed. These will provide great help to optimize current equipment and raise the efficiencies of operations and at the same time serve as a tool to design new and better equipment to increase overall recycling performance.

Call number: LD2668 .T4 1966 H436 / Master of Science

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