Floor- or ceiling-sliding for chemically active, gyrotactic, sedimenting Janus particles

Surface bound catalytic chemical reactions self-propel chemically active Janus particles. In the vicinity of boundaries, these particles exhibit rich behavior, such as the occurrence of wall-bound steady states of "sliding". Most active particles tend to sediment as they are density mismatched with the solution. Moreover Janus spheres, which consist of an inert core material decorated with a cap-like, thin layer of a catalyst, are gyrotactic ("bottom-heavy"). Occurrence of sliding states near the horizontal walls depends on the interplay between the active motion and the gravity-driven sedimentation and alignment. It is thus important to understand and quantify the influence of these gravity-induced effects on the behavior of model chemically active particles moving near walls. For model gyrotactic, self-phoretic Janus particles, here we study theoretically the occurrence of sliding states at horizontal planar walls that are either below ("floor") or above ("ceiling") the particle. We construct "state diagrams" characterizing the occurrence of such states as a function of the sedimentation velocity and of the gyrotactic response of the particle, as well as of the phoretic mobility of the particle. We show that in certain cases sliding states may emerge simultaneously at both the ceiling and the floor, while the larger part of the experimentally relevant parameter space corresponds to particles that would exhibit sliding states only either at the floor or at the ceiling or there are no sliding states at all. These predictions are critically compared with the results of previous experimental studies and our experiments conducted on Pt-coated polystyrene and silica-core particles suspended in aqueous hydrogen peroxide solutions.Comment: Total number of pages: 33, Number of figures: 18. The video files, as mentioned in the supplementary material will be provided by the corresponding author upon reques

Motion of Janus and Patchy Particles Near Various Boundaries

Colloidal particles with anisotropic surface properties are an emergent class of novel materials that have been indicated to possess a remarkable potential to still be fully exploited. Among anisotropic colloids, Janus and patchy particles with different compositions and functionalities recently have drawn significant attention in the field of colloid science. They are known as promising candidates to address challenges in various fields such as emulsion stabilization, oil recovery, drug delivery, biosensors and environmental remediation. A majority of the envisioned applications require Janus and patchy particles to operate near boundaries, i.e., a wall or a fluid/liquid interface, which significantly affect the dynamics of colloidal particles. Many studies have focused on understanding the dynamics and principles underlying the motion of active colloidal particles near various boundaries, yet a number of questions in the field remain to be answered. In this dissertation, motivated by the rich field of the dynamics of colloidal particles near boundaries, the goal is to shed light on Brownian and autonomous motion of Janus and patchy particles near a solid boundary and near various oil/water interfaces. Various boundaries used in the thesis serve as the means to significantly alter the behavior of such particles. Thus, the dynamics of both passive and chemically active Janus and patchy particles near a wall and different water/oil interfaces has been probed theoretically and experimentally. The Brownian dynamics simulation technique has been implemented to study different parameters that affect the dynamics of a Brownian Janus particles near a wall and under shear flow. In the experimental studies, various types of Janus and patchy particles have been fabricated using physical vapor deposition and the glancing angel deposition (GLAD) technique, respectively, and their trajectories near a wall and water/oil interfaces have been studied. The ability to engineer the behavior of chemically active particles has been demonstrated by precisely tuning their surface compositions, i.e., size of platinum cap leading to a preferential rotational behavior of patchy particles near a wall. Additionally, for various types of model gyrotactic (i.e., bottom heavy) self- motile Janus particles the coexistence of sliding states at both horizontal walls of an experimental cell has been investigated. Furthermore, adding motility to a suspension of passive Janus particles near a water/oil interface via introducing H2O2 as fuel to propel the particles has been shown to control the interfacial activity of amphiphilic Janus particles by preventing them from adsorbing to the interface. Last, the use of oxygen- permeable and impermeable oil/water interfaces has been used to shed more light on the propulsion mechanism of chemically active Janus particles in H2O2. Overall, the thesis contributes to a better understanding of the dynamical motion of both passive and active anisotropic particles near boundaries and provides information on how to engineer their dynamical behavior for various applications. Further, the thesis seeds new research directions such as the collective behavior of many-body Janus particles systems confined by boundaries resulting in self-assembled structures with various rheological properties and the rotational behavior of chemically active particles in porous media elucidating the motility of microorganisms in confined environments

External Ear Resonant Amplitude and Frequency of 3-7 Year Old Children

Objective: To measure external ear resonant amplitude and frequency in children (3-7 years old) and to compare with adult measures. Method and materials: The external ear resonance peak amplitude and frequency of 63 children 3-7 years old were recorded. All of the children had normal tympanogram and there was no cerumen in external auditory canal. 20 adult of 21-24 years old (10 male , 10 female) were selected in order to compare with children that had normal tympanogram. The tests included : 1-otoscopy 2- tympanometry 3-microphone probe tube test. Results: The average of resonance peak frequency for children and adult is 4200 Hz and 3200 Hz , respectively. The resonance frequency of children had significantly diffrence with average of resonance frequency in adults. The average of resonance peak amplitude for children and adult is 17.70 dB and 17.17 dB , respectively. Conclusion: Resonant frequency and amplitude affect the hearing aid prescription and fitting process and calculating insertion gain; so, this measures seem should be considered in children hearing aid fitting