A MICRO-AERODYNAMIC DECELERATOR BASED ON PERMEABLE SURFACES OF NANOFIBER MATS by E. Zussman, A.L. Yarin Faculty of Mechanical Engineering Technion – Israel Institute of Technology Haifa 32000, Israel D. Weihs Faculty of Aerospace Engineering Technion – Israel Institute of Technology Haifa 32000, Israel

Contents Motivation Nanofibers manufacturing Micro Decelerator Design Drag force Analysis Experiments Conclusions

Motivation - Smart Dust Autonomous sensor node (mote) in 1 mm 3 MAV delivery Thousands of motes (K. Pister, UCB)

Micro airborne MEMS 2003 and Beyond, Albert P. Pisano, DARPA

5 cm A pyramid-shaped platform covered with nanofiber mat

Blowing In The Wind Seeds & Fruits Dispersed By Wind

Motivation (cont’) To study the drag of permeable surfaces. Whether a permeable wing is possible ?

Typical SEM micrographs of a nanofiber and a permeable non-woven mat formed by electrospinning. Nanofibers manufacturing

Electrospinning of Polymers Volt

Electrospinning of nanofibers 30,000 Volt

SEM image of electrospun PET fibers. Fiber diameters are in the range of  m 10 micron

Aligned Fibers (diameter nm, pitch (distance between centers)  m

5 cm A pyramid-shaped platform covered with nanofiber mat. Micro Decelerator Design

L DD Payload  Sketch of the pyramid-shaped platforms. L [mm]Weight, Wplatform [g] Angle, α [degrees] Diameter, D [mm] Model No# Parameters of the pyramid-shaped platforms

A platform falling down in front of a ruler attached to the wall. g Experiments

d~ 4  m Local airflow through such openings is characterized by small values of the local Reynolds number The decelerating effect (creeping flow) of a nanofiber on the flow extends at distances of the order of the longest size involved a b Elongated Rod b<<a The drag force on the rod: F=F( ,a,U) U Drag force Analysis

The porous mat will act as an effectively intact (impermeable) surface

Comb-like wings of some insects

Falling objects weight W, is equal to the drag force imposed on them by air. Appropriate drag force model for the pyramid-shaped platforms (Schiller and Naumann) Assuming that C D is constant in a wide range of variation of Re for all the permeable and impermeable platforms we are dealing with.

Comparison of the experimental dependencies of terminal velocity on weight/drag force for permeable models 1,3, and 5.

Cumulative data on the dependence of the drag coefficient on the Reynolds number for permeable models (differ by  and  D).

Drag coefficient vs. Re for permeable models 1 and 3. The semi-vertical angles are  =63° and  =66 ° respectively.

C D vs. Re for the pyramid-shaped permeable models 2 and 5. The semi-vertical angles are  =77° and  =76 ° respectively.

C D vs. Re for permeable and impermeable model 3

C D vs. Re of two geometrical similar impermeable models 1 and 3 covered by a plastic wrap.

Summary and conclusions Terminal velocities of the permeable and impermeable model 1 were scaled as Schiller and Naumann model (payloads between 0.1 and 1.7 g) Decrease in the average hole size between the nanofibers (or porosity) achieved by their longer deposition leads to a significant increase of the drag coefficient. Permeable platforms with holes of the order of several microns (which is about ten times the nanofibers diameter) are essentially impermeable for air flow.

A platform with dihedral covered with nanofiber mat

High tensile strength stronger than steel (on weight basis ) High extensibility comparable to rubber (elasticity)