PDF download links for all papers coming soon.
Foldscope: Origami based print-and-fold paper microscope
J. Cybulski, J. Clements and M. Prakash
Submitted March 2013, under review
The hungry fly: Hydrodynamics of feeding in the common house fly PDF Link
M. Prakash and M. Steele *
Physics of Fluids, Vol. 23 (2011)
The poem by William Oldys perfectly sums up this work which started as a curious inquiry into the nature of pumps in insects. Entomologists have long described the physical layout of mechnical machines we call insects, but the dynamics of how these machines work has never been seen before. Miles and I rigged up Xray microscopy setups to image insects in all the glory (in-vivo). From what we can tell, these are some really efficient pumps.
"On a Fly Drinking Out of His Cup"
Busy, curious, thirsty fly!
Drink with me and drink as I:
Freely welcome to my cup,
Couldst thou sip and sip it up:
Make the most of life you may,
Life is short and wears away...
By William Oldys (1696-1761)
Face-selective electrostatic control of nanowire synthesis PDF Link
J. Joo, B. Chow, M. Prakash, E. Boyden, J. Jacobson
Nature Materials Vol. 10, 596-601 (2011)
Feynman in his now famous talk declared "Plenty of room at the bottom" which lead to a gold rush for making things smaller and smaller. It turned out he was right!! But before you get too excited, controlled fabrication (some call it synthesis) at the nanoscale is not an easy task. Here we show a general strategy for exploiting face-selective electrostatic adsorption of non-zinc complex ions on a growing interface to modulate zinc oxide nanowire growth; all the way from a flat pancake to a tiny spines (that is three orders of magnitude in aspect ratio change). One single parameter to tune, and all the room to play.
Interfacial propulsion by directional adhesion PDF Link
M. Prakash and J. Bush
Int. J. of Nonlinear Mechanics, Vol. 46, 607-615 (2011)
So you are sitting on a pond, watching tiny rain drops hit the surface and ripple along. It's all peace and quiet on the surface of a pond. But suddenly comes a "water strider" a cheetah of the surface tension world zipping along the water like nobody's business (~0.5 m/sec). But wait, did'nt all the scanning electron microscopy images show that the legs of a water strider (and almost all the 1800 other species) are superhydrophobic. So if you don't touch the fluid interface, how do you generate such high traction forces. I built a fluid-interface force spectroscopy setup to measure direct propulsion forces generated by individual superhydrophobic surface - and "aha" to our surprise, water strider legs exhibit unidirectional anisotropy. What that means is the surface has a prefrential direction in which a fluid contact line would happily move along one direction on a surface but have a high resistance when moving along another. This work has now inspired a large number of "unidirectional superhydrophobic surfaces" with commercial applications. But wait, water striders thought of this idea millions of years ago.
On a tweezer for droplets PDF Link
J. Bush, F. Peaudecerf, M. Prakash, D. Quere
Advances in Colloid and Interface Science, Vol. 161, 10-14 (2010)
In physics, ratchets are mechanisms that generate symmetry breaking (due to physical principles) from periodic energy landscape (or motion). A lot of them have been discoeverd, from a brownian ratchet to an optical ratchet. Here we describe a "capillary ratchet" generated by an assymetry in contact-angle hysteresis leading to unidirectional droplet propulsion. Sometimes assymetries can be so mind-bending.
Drop propulsion in tapered tubes PDF Link
P. Renvoise, J. Bush, M. Prakash and D. Quere
Euro Physics Letters, Vol. 86, 1-5 (2009)
Take a small pool of water and dip a fine glass capillary. Voila! the water rises up almost as if something is pulling it up (amd something is pulling it up). This experiment was done and basically understood in the 1600's. But now take a conical capillary. Put a drop of water and watch what happens. The drop will spontaneosuly move towards the narrow end due to the laplace pressure gradient generated due to the taper. Though this is such a simple geometrical configuration, fluid flow in a tapered tube is non-trivial. Here we derive a stability criteria for tapered tubes of all shapes and form.
Surface tension transport of prey by feeding shorebirds: The capillary ratchet PDF Link
M. Prakash, D. Quere and J. Bush
Science, Vol. 320 (5878), 931-934 (2008)
Darwin was fascinated with bird beaks (amongst other things Darwin was fasicnated with). Shape and form in biology (from the perfect beak of a bird to swimming paddle of a blue whale) evolve to optimize for function. But as with everything else in biology, it's often hard to tell how optimal is something (mathematically speaking). From an observation I made on a lake of a shore bird so fascinating (it catches your attention right away when you see it spinning in circles, all the time), I stumbled on an unusual strategy they use to transport fludis through the beak utilizing contact angle hysteresis. Usually contact angle hysteresis impedes motion, the reason why a drop of water sticks to window glass on a rainy day. Here was a case when this is the only reason the droplets are transported. We ended up calling this mode of transport a "capillary ratchet." The bird beak geometry is optimized in several species to take advantage of this physical principle. What else could have Darwin asked for - maybe a mathematical equation for his "finch" beaks.
The integument of water-walking arthropods: Form and function PDF Link
J. Bush, D. Hu, M. Prakash
Advances in Insect Physiology, Vol. 34 117-192 (2007)
Here we review the fasicntaing diversity of inscet cuticle found in water-walking arthropods, from spiders to beetles and everything in the middle. A fluid interface acts as an ecological niche so wonderful, with fasicnating adaptations from locomotion to breathing underwater. Yes breathing underwater - by diving down with tiny little surface bubbles. That's equivalent of a physical lung made out of a bubble.
Water walking devices PDF Link
D. Hu, M. Prakash, B. Chan, J. Bush
Experiments in Fluids, Vol. 43, 769-778 (2007)
So if insects can walk on water, why should we stand back. Here we attempt to make machines capable of walking on water (and succeed). Not an easy challenge considering surface tension supports very little weight. Though if you were to wear a shoe several kilometer in size, you might be able to stand on water. That is a start.
Microfludic Bubble Logic PDF Link
M. Prakash, N. Gershenfeld
Science Vol. 315, 832-835 (2007)
For the last 100 years, computation has been used as a mechanism for information processing. Even though physical laws directly enforce a necessary association of bits of information with physical entities (e.g. electrons in a microprocessor or pieces of chalk on a board), computation has not been developed as a paradigm for algorithmic assembly of physical materials. To make computation explicitly physical (literally), We invented a new logic family purely implemented in multi-phase Newtonian fluids that merge chemistry and computation, opening doors for algorithmic manipulation of entities at a mesoscale (1-100 microns). Welcome to the world of tiny little bubbles zipping in fluidic networks talking to each other (hydrodynamically speaking) implementing functions you desire.
N. Gershenfeld, M. Prakash
Telektronikk Vol. 3, 22-26 (2004)
Inspired by the open wireless revolution, we developed a process where anybody could "print" high gain antennas for wireless devices and paste it on the window glass pane. DIY high-gain antenna that costs a couple cents. That's got to be good for something.