Bubble raft movies (videos) and pictures

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Cracks in "brittle" materials

Cracks in ductile materials

Multilayers Machining Interacting dislocations Pictures of crystal defects: dislocations, vacancies, divacancies, subgrain boundaries Support People who contributed to this work Defects in bubble rafts HOME
How to make a bubble raft

These videos and images are copyrighted; feel free to copy and use them for whatever purpose you wish but don't remove copyright.


What is a bubble raft?


A bubble raft is an array of soap bubbles that models the structure and dynamics of a real crystal consisting of atoms or ions.  Most bubble rafts consist of a single layer and are therefore 2-dimensional; even so, they exhibit many of the same kinds of phenomena as seen in 3-dimensional crystals.  The bubbles in a bubble raft are typically about 1 mm in diameter.  If all the same size, the bubbles will self-assemble into a hexagonal array, the same as the arrangement of atoms in the {111} planes of a face centered cubic metal such as copper of aluminum.  Bragg and coworkers invented bubble rafts around 1940, and ever since then scientists have used bubble rafts to help model defects in crystals, especially dislocations. My students have learned how to make bubble rafts having other kinds of crystal structures and different properties (the "rocksalt" or "brittle" rafts, below) by combining bubbles of different sizes. 

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"Brittle" or "rocksalt" structure bubble raft


Vacancy and divacancy (relaxed)


Machining or metal cutting


Dislocation and other defect structures


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Defects in bubble rafts:

The properties of crystals are often dictated by the behaviors of their defects.  Bubble rafts contain a number of crystal defects including dislocations (responsible for plastic deformation), grain boundaries, vacancies (missing atoms), and impurity atoms.  The edge of a bubble raft, analogous to the surface of a solid crystal, is a crystal defect. 

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How to make a bubble raft: 


Water (deionized, reverse osmosis, or distilled water; you don't want minerals to reduce the efficacy of the soap)

Clear, liquid dish soap (some dish soaps work better than others)


Hypodermic needle, sharp end ground flat

Fish tank regulator (air pump) and tube

Shallow pan (e.g., 9"x13" baking dish)

Narrow rubber stopper slit partway along its length so that it can hold the needle at depth in the water; needle should be able to stick out from end

Adhesive for gluing the stopper onto the bottom of the pan and sealing hypodermic needle into tube



For the soap solution use water (reverse osmosis, deionized, or distilled better than tap) and dish soap without lots of additives (I prefer Dawn® soap; some formulations are better than others) and glycerin, which helps the bubbles to last longer. The exact formulation for the soap solution isn't critical; I use something like 50 ml soap in 1 liter of water (play around with the concentration yourself!). A shallow tray such as a baking pan can hold the soap solution, which should be about 2-3 cm deep. If the pan is transparent you can place the tray on an overhead projector and project the bubble raft onto a screen.  A tray made from plexiglass works great.  Alternatively, if you want to make movies and take photographs an inexpensive metal baking pan works fine.  Place a blackened (exposed) transparency slide at the bottom of the pan after you have introduced the soap solution to help improve contrast of the bubbles in a photograph or movie.  You then need to blow bubbles through a tube with a hypodermic needle at the end (grind off the sharp tip).  To control air pressure use a fish tank regulator plugged into a Variac.  5-20 volts should work.  To help make sure bubble diameter is uniform, you want to maintain the needle undisturbed, at a constant depth in the soap solution.  I do this by gluing a narrow rubber stopper onto the bottom of the pan, cutting a slit into the stopper, and inserting the needle into the slot.  The bubbles need to be blown to the side as they come to the surface; otherwise they will make a 3D foam. 

bubble raft schematic

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People who contributed to this work: 

Mike Starr, Maria del Carmen Lopez Garcia, Ryan Webster, Walt Drugan, Wendy Crone, Chris Kailhofer, MSE 361 class

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This research was supported by the National Science Foundation, Mechanics and Materials Program, Grant CMS-9800157, the National Science Foundation Materials Research Science and Engineering Center at UW-Madison, Grant DMR-0520527, and the National Science Foundation Civil, Mechanical, and Manufacturing Innovation Division, CMMI-0824719.



Bubble raft models of cracks in brittle materials

(Mike Starr, Ryan Webster)

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The hallmark of a brittle material is that a microscopic flaw (crack) will tend to elongate rather induce plastic deformation when a stress is applied. In a crystalline solid this deformation is accomplished by the emission of dislocations from the crack, in which case the crack becomes  more blunt or rounded. The tendency for brittle behavior is more pronounced with longer cracks. Do you see dislocations being emitted from crack tips in these bubble rafts?  Which of these cracks behaves in a brittle manner? 



MOVIES: Bubble raft models of cracks in ductile materials

(Mike Starr, Maria Lopez Garcia)


Notice how these bubble raft cracks tend to emit dislocations and become more blunt as the raft is stressed. 

MOVIES: Bubble raft models of cracks in multilayers and near surfaces

(Mike Starr, Maria Lopez Garcia)

Mike Starr created these bubble raft models to examine fracture in nanolayered materials.  Different layers are distinguished by different bubble size.  These cracks and the dislocations emitted by them are heavily influenced by the presence of the nearby interfaces. Dislocations undergo many interesting interactions including interactions with interface dislocations. Vacancies are created by dislocation interactions, and sometimes further interactions lead to the nucleation and growth of voids. 


MOVIES: Bubble raft models of interacting dislocations

(Mike Starr, MSE 361)


An individual dislocation is distinguished by its Burgers vector (defined in the text accompanying the pictures, below).  In a hexagonal bubble raft there are six different Burgers vectors, one for each direction in the lattice.  Two dislocations can combine to make a third dislocation.  The Burgers vector of the third dislocation is the sum of the first two. 

In a bubble raft a vacancy is equivalent to two dislocations of opposite sign separated by a single atomic "plane" (or line of atoms in 2D) and bound together by elastic interaction.  These dislocations can dissociate under external stress or in the presence of a nearby dislocation. This happens frequently in movies throughout this collection. 

Dislocations interact with surfaces (i.e., the edges of the rafts) and grain boundaries. A free surface attracts dislocations.  A rigid surface (plexiglass placed against the edge) tends to repel dislocations.  However, because the boundary between the plexiglass and bubble raft is slippery, a dislocation which gets sufficiently close to the plexiglass can interact by dissociating into two other disocations, one of which will travel parallel to the plexiglass.  Normally this dissociation is unfavorable because the energy of two dislocations is higher than that of a single dislocation; nevertheless, in the dynamics of a bubble raft the reaction can take place. 

Grain boundaries can emit and absorb dislocations. 

These movies show some neat dislocation interactions.


MOVIES: Bubble raft models of machining; metal cutting (Mike Starr)

These movies illustrate the cutting of a ductile material by wedge-shaped tools, similar to a metal cutting or machining operation. Notice how this cutting is accompanied by the emission of dislocations from the interface between bubble raft and cutting tool.  These dislocations allow the bubble raft to form a "chip" of material that peels away from the bulk.  There are many interesting interactions, and one could spend considerable time analyzing all of them.   

Depending on the angle and bluntness of the cutting tool the dislocation emission and resulting structure will differ.  The portion of the raft that forms the "chip" likes to line up crystallographically so that one of its hexagonal directions lies parallel to the interface with the cutting tool.  (Apparently this configuration is a low energy boundary between the raft and cutting tool).  For the 30° tool the hexagonal crystal doesn't have to change orientation for this to happen.  In contrast, for the 45° tool the crystal must re-orient, so a line of "geometrically necessary" dislocations forms that separates the two different crystal orientations.  This line of dislocations lies parallel to the interface with the cutting tool and is similar to the low angle grain boundary structures shown in the bubble raft pictures below. 



Bubble raft models of crystal defects from my MSE361 class



Subgrain boundaries are grain boundaries with misorientation small enough that individual dislocations can be resolved.  A symmetric tilt boundary is a boundary where the crystal on either side is rotated symetrically away from the plane (or line in a 2D crystal) of the boundary.  The boundary is comprised of a single family of dislocations stacked on top of each other.

An asymmetric tilt boundary is one in which the crystal rotation away from the boundary is different on either side. An asymmetric tilt boundary is comprised of two families of dislocations.

A divacancy is two nearby vacancies bound to each other by some form of attraction. In the image shown below the divacancy has relaxed into a low energy configuration.

A phase boundary is the boundary between two crystals with different structure or composition. For the most part, in our bubble rafts the different phases are represented by regions with different bubble sizes. A coherent phase boundary would be a boundary for which all the atoms on either side of the boundary line up across the boundary. This means that if the atoms on either side of the boundary are of different size, then the two lattices have to distort for the alignment to take place. This can only happen if at least one of the crystals is extremely thin, having a thickness on the order of a few tens of atomic spacings in real crystals. Otherwise, dislocations will form along the phase boundary to take up the mismatch between crystals, leading to semicoherent boundaries. These mismatch dislocations produce all kinds of interesting effects and are of central importance in many areas of technology ranging from high strength alloys to microelectronics. Phase boundary dislocations can be seen in the image below, They can also be seen moving and interacting with lattice dislocations in many of the multilayer fracture movies, above.

A dislocation, shown below (use the hyperlinks to see images), is a particular type of topological defect in the crystal.  Wikipedia has a description of dislocations in 3D crystals; dislocations in bubble rafts are like 3D edge dislocations in cross-section. 

Here is what defines a dislocation:  If a path is traced along rows of atoms in such a way that the path would ordinarily close in a perfect crystal, then that same path won't close if it contains a dislocation.  In a hexagonal crystal this path could be a parallelogram or hexagon traced along lines of atoms, and it should be big enough to easily enclose the dislocation but not contain two dislocations.  This path is called the Burgers circuit, and if there is no dislocation is inside it, the beginning and end of the path will be the same because the path closes on itself.  In case the Burgers circuit doesn't close, you know there is at least one dislocation contained inside.  The Burgers vector, or topological property of the dislocation, is the vector that points from the beginning of the path to the end of the path. In one of these hexagonal rafts the Burgers vector should correspond to an interatomic separation, or distance between centers of bubbles.

Vacancies don't have Burgers vectors (or, equivalently, their Burgers vectors are zero).  Even though vacancies in 2D rafts are equivalient to pairs of dislocations (see interacting dislocations, above), the Burgers vectors of the two dislocations are equal and opposite and cancel out in the Burgers circuit analysis. Also, vacancies in bubble rafts don't move under stress unless their dislocations dissociate, in which case the dissociated dislocations move off in opposite directions. 

Many of the movies above show moving dislocations.  The movies that best illustrate this process up-close are the ones with mode-II crack loading