Dr. Dunn conducts
his research in the Particle
Dynamics Laboratory at the Hessert
Laboratory for Aerospace Research.
Current Research
ApproachIn the Particle Dynamics Laboratory at the University of Notre Dame we investigate problems related to the dynamics of small (~1-200 µm diameter), electrically neutral or charged droplets and particles in various pressure environments. Our efforts primarily are experimental. To this end, we have developed a laboratory equipped with state-of-the-art equipment to visualize and measure droplet/particle characteristics under both atmospheric and vacuum conditions. We emphasize conducting detailed and accurate experiments that provide a thorough data base from which models of the underlying physical processes can be assessed. Where appropriate, we develop analytical or numerical models.
Microparticle Deposition onto Surfaces
We began studying microparticle dynamics in 1987, when we investigated how to levitate and disperse electrostatically-charged, micron-size particles under atmospheric and vacuum conditions. This was a combined experimental and analytical study. We developed and characterized the performance of an electrostatic solid particle generator and measured its effluent particle size and velocity distributions. We then developed an analytical model of particle impact and adhesion with surfaces that could relate to such a system. We extended this model to include the impact of electrically charged microspheres with surfaces under space vacuum conditions. We also conducted experiments in which individual particle incident and rebound velocities and charge transfer were measured accurately under atmospheric and vacuum conditions. Lastly, we have investigated the influence of the microsphere's spin and impact angle on its rebound characteristics from surfaces having varying degrees of roughness.
Our research in this area recently has been funded by two agencies, the Electric Power Research Institute (EPRI), from 7/93 until 7/99, entitled "Combined-Effects Aerosol Experiments", and the Center for Indoor Air Research (CIAR), from 7/96 until 6/99, entitled "Particulate Deposition onto Surfaces". During the initial phase of the EPRI-funded project (until 7/96), our research was directed primarily to obtain experimental data during aerosol processes involving several simultaneously acting transport and deposition mechanisms. We focused on investigating one process in detail, that of microsphere deposition onto a flat surface. Currently, during the second phase of this project, we (in collaboration with Professor R. Brach) are developing analytical and numerical models of the deposition process and validating these models with our experimental results. The CIAR-funded project (in collaboration with Professor R. Brach) involves both experimental and analytical efforts to investigate the deposition of naturally occurring microparticles onto realistically rough surfaces.
At a fundamental level, the CIAR project considers what actually happens during surface contact. It also addresses several practical issues including how to model attachment and rebound for a broad range of particle shapes, environments, surface and particle materials. The validation and exploratory experiments will be conducted in the Particle Dynamics Laboratory using laser diagnostics to measure a single microparticle's velocity components and size before and after impact. Micro-video-photography of the microparticle during surface contact also is planned. The project further will consider the dominant forces involved in microparticle attachment which include van der Waals (molecular), electrostatic and capillary forces and how these forces change as the microparticle shapes and surface roughness become more accentuated. The ultimate goal of this research is to produce an experimentally validated model of how indoor air contaminants such as smoke particles, pollens and spores adhere to room surfaces.
Microparticle Resuspension and Transport from Surfaces
The most recent research area under investigation in our laboratory is microparticle detachment and entrainment from surfaces by fluid flow. This program (funded partially by Philip Morris) has both experimental and theoretical aspects. Experimentally, we are using microphotography, laser light-sheet, phase Doppler and hot-wire anemometry in our 8 in. by 8 in. cross-sectional area aerosol wind tunnel to fully characterize the motion of an individual microparticle during detachment and its subsequent motion in the air. We have acquired data to characterize the progress of detachment with time, flow velocity, and friction velocity on monodisperse and polydisperse microparticles of different sizes and shapes. The substrate is scanned using an Atomic Force Microscope (AFM) in order to model the reduction in smooth pull-off force due to surface roughness. Observations made with our video microscopy show that the detachment occurs through motion along the surface rather than through lift-up (this is supported theoretically). Consequently, detachment is a necessary but insufficient condition for entrainment. We also have measured a threshold of initial number density above which surface collision mechanisms significantly reduce the flow velocity required for detaching most of the microparticles. Observations show that the detachment occurs through both short-term and long-term regimes. The former has a much higher detachment rate than the later. Transition between the two regimes occurs when flow acceleration ceases. The velocities of the microparticles after detachment and their rotation rates are measured using a strobed laser light-sheet. The causality between turbulent bursts and detachment when the microparticles are totally embedded within the viscous sublayer is studied. Theoretically, a force balance approach is used to predict the onset of detachment. The model predicts that detachment occurs when the moment of the drag force overcomes the moment of the pull-off force.
70 mm diameter microspheres detaching from a glass surface as the flow velocity is increased (from left to right).
Different microparticle views achieved in our experimental set-up.
Electrohydrodynamic (EHD) Atomization of Liquids
We also have studied the EHD atomization of liquids from single, two and multiple parallel liquid streams, and the resulting process of droplet interaction. Our research, beginning in 1987, involved measuring droplet velocity and size distributions in the interacting spray produced by two parallel EHD sprays at high charge densities. We acquired detailed data on the droplet field and identified under what conditions droplets from adjacent could intermix. We then directed our efforts to determine the droplet characteristics of an individual EHD spray and to relate these results to various models of the droplet charging process. The application of a laser diagnostic to measure droplet speed in the developing region of such a spray also was investigated. Our most recent efforts have examined how a small perturbation in the local electric field can enhance droplet mixing within an individual EHD spray. We also have developed a numerical model for droplet motion within the EHD spray, which successfully predicts the observed droplet motion. Other issues related to this research area that we have addressed include the interaction between electrically charged falling, droplet pairs and the single and double periodicity of droplets emanating from interconnected orifices.
(JPEG - 14K)
Characterization
of Space Dust through Measurement and Modeling
This is a collaborative research project between the Particle Dynamics Laboratory in the Center of Flow Physics and Control at the University of Notre Dame and the Osservatorio Astronomico Di Capodimonte in Naples, Italy, as part of a European Space Agency> mission. The focus of this research is to develop instrumentation that can be used to detect and analyze cosmic and planetary dust in space. The calibration of the instrumentation and the modeling of dust behavior in the sampling environment and within the sampler is considered also.
Experiments are being conducted in the Particle Dynamics Laboratory to ascertain the capability of a dispensing a Martian soil simulant by electrostatics. This method will be used to calibrate an instrument developed to detect and measure the incident velocity of cosmic dust. A Martian soil simulant, JSC Mars (<20 µm), was selected to calibrate the system. The soil is composed of grains on the order of 10 µm, which often exist in agglomerates as large as 450 µm, as shown in the figure.
The agglomerates were levitated in the presence of a relatively weak electric field using an existing electrostatic microparticle dispenser developed by Olansen, Dunn, and Novick in 1989. The dispenser consists of a central, energized mesh screen on top of which a plate holding the particles rests. Plates on either side of the central electrode are grounded, thus establishing an electric field between the plates. The dispenser operates by transferring a positive charge to the particles while they are resting on the energized plate. When repulsive forces acting on the particles overcome their adhesion forces (and gravity for larger particles), the particles move from the energized plate to the grounded plate. Once the particles' charge is lost to the grounded plate, they are attracted back to the energized electrode. The particles continue to “bounce” between the electrodes until they exit out of a hole in the bottom plate. While the particles are loosely directed to the exit, it is the random motion of the particles that leads to their eventual dispensing.
A new dispenser has been designed and is now being operated in a vacuum so higher electric field intensities can be reached. If higher electric fields can be applied to the particles, the dispensing of single grains should be possible. There are only two electrodes so as to minimize the distance the particles need to travel to exit the dispenser. Because the particles are initially placed on the same electrode in which there is the exit hole, the exit hole was raised so as to prevent particles from accidentally falling out. Finally, the insulating wall runs at an angle, focusing the electric field lines towards the center of the top electrode, and therefore over the exit hole. This new design will be tested in both air and in a vacuum.
The design of space dust sampling systems is underway and aimed toward reducing a system's size to MEMs scale. This system would capture and transport micrometer-sized space dust, and then analyze its size and mass distributions and number concentration. Modeling done to support this effort involves the transport, deposition and resuspension of electrically charged microparticles in the presence of electric fields in a vacuum environment.
(click for larger view)
Martian soil simulant
Microcontroller-Based Data-Acquisition Systems
We are working
to develop a miniature, microcontroller-based data-acquisition system to be
used for remote, autonomous data-acquisition. Currently, the primary focus
of this project, funded by a General Electric Multidisciplinary Learning Module
Grant, is to develop a simple system that will be used to introduce undergraduate
engineering students to the fields of data acquisition, data reduction, and
electronics, specifically microcontrollers. The system will also be used in
several other undergraduate aerospace, mechanical, computer science, and electrical
engineering courses.
The custom-made microcontroller based system currently being developed consists
of a microcontroller with eight 12-bit A/D channels and two serial communication
ports, a 64 kB EEPROM memory chip, and a custom-made 1100 mAh, rechargeable
NiMh battery pack, which will be capable of supplying power for continuous
use of up to 6 hours.
Students can use this system to conduct predefined experiments or to investigate their own curiosities. Because of the system’s small size, low weight, and freedom from wires that tether computer-based data acquisition systems, a broad range of interesting and informative "real-world" experiments can be performed. Using similar systems, applications to date have included studying the accelerations acting on a swinging pendulum, measuring acceleration and velocity profiles during a model rocket launch, and determining the temperature profile of a nearby lake using a remote control boat. More information can be found at http://www.nd.edu/~tszarek/research/.
(Click image for larger view) View Rocket
Movie 4.4 mpg
A. Ibrahim, P.F. Dunn and R.M. Brach, "Microparticle Detachment from Surfaces Exposed to Turbulent Air Flow: Effects of Several Controlled Factors", accepted for publication, Journal of Aerosol Science, December 2003.
W. Cheng, P.F. Dunn and R.M. Brach, "Contact between a Smooth Microsphere and an Anisotropic Rough Surface", Journal of Adhesion, Vol. 79, pp. 749-776, 2003.
A. Ibrahim, P.F. Dunn and R.M. Brach, "Microparticle Detachment from Surfaces Exposed to Turbulent Air Flow: Controlled Experiments and Modeling", Journal of Aerosol Science, Vol. 34, pp .765-782, 2003.
W. Cheng, P.F. Dunn and R.M. Brach, "Surface Roughness Effects on Microparticle Adhesion", Journal of Adhesion, Vol. 78, pp. 929-965, 2002.
W. Cheng, R.M. Brach and P.F. Dunn, "3D Modeling of Microsphere Contact/Impact with Smooth, Flat Surfaces", Aerosol Science and Technology, Vol. 36, pp. 1045-1060, 2002.
A. Ibrahim, R.M. Brach and P. F. Dunn, "Microparticle Detachment from Surfaces Exposed to Turbulent Air Flow: Microparticle Motion after Detachment", accepted for publication, Journal of Aerosol Science, May 2004.
A. Ibrahim, P.F. Dunn, and R.M. Brach, "Microparticle Detachment from Surfaces Exposed to Turbulent Air Flow: Effects of Flow and Particle Deposition Characteristics", Journal of Aerosol Science, Vol. 35, pp. 805-821, 2004.
R.M. Brach, P.F. Dunn and X. Li, "Experiments and Engineering Models of Microparticle Impact and Deposition", pp. 227-282 in Particle Adhesion: Applications and Advances, Taylor & Francis, New York, 2001.
X. Li, P. F. Dunn and R.M. Brach, "Experimental and Numerical Studies of Microsphere Oblique Impact with Planar Surfaces", Journal of Aerosol Science, Vol. 31, pp. 583-594, 2000.
X. Li, P. F. Dunn and R.M. Brach, "Lycopodium Spore Impact onto Surfaces, Atmospheric Environment", Vol. 34, pp.1575-1581, 2000.
R.M. Brach, P.F. Dunn and X. Li, "Experiments and Engineering Models of Microparticle Impact and Deposition", Journal of Adhesion, Vol. 74, pp. 227-282, 2000.
R.M. Brach, X. Li and P.F. Dunn, "Parameter Sensitivity in Microsphere Impact and Capture", Aerosol Science and Technology, Vol. 32, pp. 559-568, 2000.
Direct
comments, questions, and corrections to amedept@nd.edu