Abstract: The phenomenon of heterogeneous nucleation in the gas phase is briefly introduced, as well as a related instruments referred to as a condensation particle counter (CPCs). CPCs are widely used to detect single nanometer particles and molecular ions, by growing them into visible sizes. We review prior use of sheathed CPCs, where nanoparticles are exposed to a well-defined supersaturated state by being injected into the center of a larger “sheath” flow of air saturated with a vapor. An original member of this sheathed CPC class is the Variable Supersaturation Condensation Particle Sizer (VSCPS) of Gallar et al. (2006). A slight variant of this device is reexamined here with vapor of 1-butanol and an aerosol of highly uniform singly charged polyethylene glycol particles (diameter d_p=3-9 nm), produced by a bipolar electrospray, and size-selected via mobility separation. These particles sense an almost uniform maximal saturation ratio S_max = C w_, controlled linearly with the wet fraction w= Q_sat/(Q_sat+Q_dry), by mixing two flows, Q_dry and Q_sat of dry and saturated air.  We find substantially steeper activation probability curves P(w) than previously observed with any CPC.

Basic heterogeneous nucleation studies require knowledge of S_max, hence the constant C in the relation S_max = C w. Here we find C by assuming that classical heterogeneous nucleation theory with perfect wetting applies at the largest particle sizes. This choice of C fixes also the theoretically unspecified preexponential term K governing the nucleation rate. This results in excellent agreement between data and theory for the size dependence of S_max(d_p) at P(w)=0.5 at all sizes studied. A very good agreement is also found for the entire activation probability curves P(S_max). This is the first successful confirmation of classical heterogeneous nucleation theory to become available.      

Gallar, C. A. Brock, J. L. Jimenez, C. Simons, A Variable Supersaturation Condensation Particle Sizer, Aerosol Sci. & Techn. 40 (6) (2006) 431–436.

Biographical Sketch: Dr. Juan Fernandez de la Mora graduated from the School of Aeronautical Engineering (Madrid, 1975) and received a PhD from Yale’s School of Engineering (1981). After a postdoctoral stage at UCLA, he joined the Yale faculty in 1981, where he became a Professor of Mechanical Engineering in1992. He has worked on the kinetic theory of gas mixtures, the structure of electrified liquid cones, and the separation of nanoparticles in the gas phase by inertial, electrical and condensation phenomena. He has co-authored nearly close to 200 articles and 14 US patents.

Abstract: Within the past decade, significant progress has been made in using quantum computing (QC)  for solving classical problems. In this talk, an overview is made of the ways by which QC has shown promise for fluid dynamics and combustion research. This is via both quantum-ready and quantum-inspired algorithms. The former deals with problems that either have the potentials to benefit from quantum speed-up on universal gate-based digital computers, or those that can be solved on quantum simulators. The latter deals with new classical algorithms that have emerged from many-body quantum physics.

Biographical Sketch: Dr. Peyman Givi is Distinguished Professor and James T. MacLeod Professor of Mechanical Engineering and Petroleum Engineering at the University of Pittsburgh. Previously he held the position of University at Buffalo Distinguished Professor of Aerospace Engineering. He received Ph.D. from the Carnegie- Mellon University (PA), and BE from the Youngstown State University (OH).

Abstract: Inflating a rubber balloon leads to a dramatic shape change: a property that is exploited in the design of soft robots and deployable structures. On the one hand, fluid-driven actuators capable of complex motion can power highly adaptive and inherently safe soft robots. On the other hand, inflation can be used to transform seemingly flat shapes into shelters, field hospitals, and space modules. In both cases, just like the simple balloon, only one input is required to achieve the desired deformation. This simplicity, however, brings strict limitations: soft actuators are often restricted to unimodal and slow deformation and deployable structures need a continuous supply of pressure to remain upright. Here, we embrace multistability as a paradigm to improve the functionality of inflatable systems. In the first part of this seminar, I exploit snapping instabilities in spherical shells to decouple the input signal from the output deformation in soft actuators–a functionality that can be utilized to design a soft machine capable of jumping. In the second part of the seminar, I draw inspiration from origami to design multistable and inflatable structures at the meter scale. Because these deployable systems are multistable, pressure can be disconnected when they are fully expanded, making them ideal candidates for applications such as emergency sheltering and deep space exploration. Together, these two projects highlight the potential of multistability in enabling the design and fabrication across various scales of multi-form, multi-functional, and multi-purpose materials and structures.

 

Biographical Sketch: Katia Bertoldi is the William and Ami Kuan Danoff Professor of Applied Mechanics at the Harvard John A.Paulson School of Engineering and Applied Sciences. She earned master degrees from Trento University (Italy) in 2002 and from Chalmers University of Technology (Sweden) in 2003, majoring in Structural Engineering Mechanics. Upon earning a Ph.D. degree in Mechanics of Materials and Structures from Trento University, in 2006, Katia joined as a PostDoc the group of Mary Boyce at MIT.  In 2008 she moved to the University of Twente (the Netherlands) where she was an Assistant Professor in the faculty of Engineering Technology. In January 2010 Katia joined the School of Engineering and Applied Sciences at Harvard University and established a group studying the mechanics of materials and structures. She is the recipient of the NSF Career Award 2011 and of the ASME’s 2014 Hughes Young Investigator Award. She serves as Editor for the journals Extreme Mechanics Letters and New Journal of Physics. She published over 150 peer-reviewed papers and several patents. For a complete list of publication and research information: https://bertoldi.seas.harvard.edu/. Dr Bertoldi’s research contributes to the design of materials with a carefully designed meso-structure that leads to novel effective behavior at the macroscale. She investigates both mechanical and acoustic properties of such structured materials, with a particular focus on harnessing instabilities and strong geometric non-linearities to generate new modes of functionality. Since the properties of the designed architected materials are primarily governed by the geometry of the structure (as opposed to constitutive ingredients at the material level), the principles she discovers are universal and can be applied to systems over a wide range of length scales.

Abstract: Natural phenomena, such as growth, instability, and failure, can be highly dependent upon activation of stochastic mechanisms at the microscale, such as the existence of microscopic imperfections, the action of molecular motors, and the diffusion of constituents. Yet, at the macroscale, astonishing order is often observed. In this talk, I will discuss our recent attempts to bring a deterministic understanding to explain such processes by focusing on the growth of bodies under confinement of an embedding soft matrix. Theoretical models will be complemented by experimental observations at different scales. At the small scales we exploit the growth of biofilm forming bacterial colonies and liquid-liquid phase separation, to examine the influence of confinement in determining the observed morphological transitions; at larger scales Volume Controlled Cavity Expansion (VCCE), via needle induce fluid injection, allows us to study local material properties and the transition between cavity expansion and fracture. 

Biographical Sketch: Tal Cohen is an Associate Professor at MIT. She joined the Department of Civil & Environmental Engineering in 2016 and has a joint appointment in the Department of Mechanical Engineering. She received both her MSc and PhD degrees in Aerospace Engineering at the Technion in Israel. Following her graduate studies, Tal was a postdoctoral fellow for two years at the Department of Mechanical Engineering at MIT and continued for an additional postdoctoral period at the School of Engineering and Applied Sciences at Harvard University. She received the ONR young investigator award and the NSF CAREER award in 2020, and the ARO young investigator award in 2019. Earlier awards include the MIT-Technion postdoctoral fellowship, and the Zonta International Amelia Earhart Fellowship. Her research is broadly aimed at understanding the nonlinear mechanical behavior and constitutive sensitivity of solids. This includes behavior under extreme loading conditions, involving propagation of shock waves and dynamic cavitation, material instabilities, and chemo-mechanically coupled phenomena, such as material growth. 

Abstract: Skin of fast swimming shark species such as Mako are packed with overlapping micro-scale denticles where each denticle is covered with 3-7 ribs. These textures allow sharks to swim faster than other animals in the ocean. Inspired by this capability, two-dimensional symmetric and periodic textures have been considered for the purpose of drag control and reductions of between 7-10% have been reported. Previous research on 2D textures have focused on the effect of the height and spacing of the grooves on the flow and concentrated on V-grooves (triangular grooves). However, the cross-sections of the ribs on shark denticles are concave and the few reported experiments and simulations of textures with curved profiles show that the response of these surfaces cannot be explained as a function of height and spacing alone, and other geometric features play important roles. In addition, 2D textures are simplified models of the shark scales, missing the effect of the overlaps among the denticles.

In this talk, I will examine the effect of the geometric profile of the cross-sections of 2D textures aligned in the flow direction in two cases: first in a small-scale internal flow (Taylor-Couette) and then in a larger scale external flow (boundary layer) setting. I will present the results of the experiments performed using textured covered rotors in a Taylor-Couette cell in the Couette Flow and early transition to Taylor vortex regimes, as well as textured flat samples in a water tunnel in high Reynolds number laminar flows. The custom-designed experiments involve a combination of load/torque measurements parallel with particle image velocimetry of the flow in the vicinity of the textures. I will explore the response of different profiles, and the effect of convex vs. concave cross-sectional shapes, as well as overlaps, on the ability of textures in altering the flow field, frictional loading, and flow instabilities as a function of the geometric features and flow dynamics (i.e. the Reynolds number). I will show that, overall, when compared with the well-known V-grooves, concave profiles (similar to the cross-section of the shark ribs) with height-to-half-spacing less than or equal to unity can enhance the drag reducing ability of textures while convex textures reduce the level of drag reduction.

Biographical Sketch: Shabnam Raayai is a Rowland Fellow and principal investigator at Rowland Institute at Harvard University where her lab is focused on the study of flow around textured and complex geometries. Prior to her current role, she was a postdoctoral associate at the department of civil and environmental engineering at MIT. She received her SM and PhD in mechanical engineering from MIT and have won multiple awards including the outstanding teaching assistant award from the department of mechanical engineering at MIT and Andreas Acrivos Dissertation Award in fluid dynamics from the American Physical Society.