This work was supported by the NSF under award MCB-1330685. DWM acknowledges partial support from Cornell University&#39;s Molecular Biophysics Training Grant (NIH T32GM0082567).

CAUTION: Please consult all relevant material safety data sheets (MSDS) before use. Please use all appropriate safety practices when using compressed gases, including appropriate calibrated gas handling regulators and valves, and approved gas tubing. Contact with liquid cryogens can cause&#160; severe frostbite and necrosis. Use appropriate personal protective equipment (face shield, gloves, lab coat, full length pants, closed-toe shoes), all of which must be impermeable to liquid nitrogen. Remain standing and ensure an unobstructed exit path from the apparatus when using liquid cryogens. Be aware of asphyxiation hazards when using compressed gases and liquid cryogens, and work in a well-ventilated area with adequate make-up air (a fume hood or a high air turnover rate room.)
1. Preparation of Aqueous Solutions for Density Measurements
NOTE: Because weights are more easily measured to a high accuracy than volumes, solution concentrations are measured in w/w units. All densities and melting or boiling temperatures assume an atmospheric pressure of ~100 kPa. The following steps describe the preparation of a 35% w/w glycerol solution. The same procedure can be used for other concentrations and solutes.
<ol>
	<li>For each solute type and concentration of interest, roughly estimate the required solute mass to obtain the desired final concentration (e.g., between 25% w/w and 100% w/w) for a total solution volume, Vtot = 10 mL solution volume. For example, for a 35% w/w glycerol solution (&#961;s = 1.26 g/mL), the solute mass, ms is: 
	<img alt="Equation" src="/files/ftp_upload/55761/55761eq1.jpg" /> 
	where x is the solute mass fraction (0.35) and &#961;w= 1 g/mL is the density of water.</li>
	<li>Place a 15 mL centrifuge tube (or other volume-calibrated water-impermeable container) on the pan of an analytical microbalance. Dispense the desired mass of solute/cryoprotectant (e.g., 3.77 g glycerol for a 35% w/w solution) into the tube, and record the actual solute mass.</li>
	<li>Add high purity (&#62;18 M&#937;) deionized water to bring the total mass up to 10.0 g</li>
	<li>Vortex the container for 30 s (for liquid solutes) or 5 min (for solid solutes), until the solution is optically homogeneous.</li>
	<li>Measure and record the final mass of the solution. Seal the container with an airtight cap and store at constant temperature (293-298 K).</li>
</ol>
2. Preparation of the Sample Cooling Chamber
<ol>
	<li>Place the experimental apparatus described below in an enclosure and flow dry air (&#62;5% relative humidity (r.h.)) into the enclosure. 
	NOTE: The enclosure may be a simple metal frame with its top and three sides sealed with clear plastic sheet and with experiment access via a fourth side covered with flexible plastic sheet. Wear face shields and full body covering to minimize moisture introduced by the experimenter. Moisture condensation and ice formation can interfere with cryogenic temperature density measurements in several ways, and so it must be minimized.</li>
	<li>Place a disk of neoprene rubber on the bottom of a 4.5 L glass Dewar flask, to protect the Dewar flask from damage.</li>
	<li>Carefully insert a high thermal conductivity copper chamber (a hollow cylinder with a sealed bottom) into the flask until it rests on the rubber disk. Adjust struts projecting outward from the chamber to the Dewar walls so that the chamber is centered and has no tendency to rock. 
	NOTE: The Dewar flask will hold liquid nitrogen, and the much smaller volume copper chamber will hold a liquid N2-Ar mixture. The liquid nitrogen provides a thermal bath that maintains the copper chamber and its contents at a constant temperature of 77 K and reduces boiling and evaporative losses in the chamber. The chamber&#39;s small diameter suppresses surface waves that can interfere with buoyancy measurements, and helps isolate the liquid within the chamber from frost and ice formed elsewhere in the apparatus.</li>
	<li>Insert the outlet of a gas tube with dry N2 gas flowing at ~2 L/min down to the bottom of the copper chamber, and purge the chamber of moist air.</li>
	<li>Slowly pour liquid nitrogen into the Dewar flask, outside the copper chamber, allowing time for nitrogen boil-off. 
	NOTE: The final fill level, after boiling has ceased, should be within approximately 4 cm of the top of the copper chamber.</li>
	<li>Cover the outer portion of the Dewar flask with an annular foam insulating lid. Remove the dry N2 gas purge tube from the copper chamber and insert into a matching opening in the lid. 
	NOTE: The combination of N2 gas from boil-off in the Dewar flask and from the purge flow expels any moist air and prevents it from condensing and crystalizing on cold surfaces.</li>
	<li>Slowly pour liquid nitrogen into the copper chamber. The final fill level, after boiling has ceased, should be within approximately 4 cm of the top of the copper chamber.</li>
	<li>Place a thin, optically transparent plastic sheet over the central opening or vent in the lid, and reduce the N2 gas flow rate to ~0.2 L/min, leaving a slight overpressure of N2 gas within the gas spaces above the cryogenic liquids. 
	NOTE: As long as cryogenic liquids are present in the Dewar and chamber, continue adjusting the N2 gas flow as needed to maintain this overpressure and prevent moisture from entering into the Dewar and forming ice.</li>
</ol>
3. Determination of the Volume and Density of the Test Mass at T = 298 K and T = 77 K
<ol>
	<li>Determine the apparent mass of a ~1 g, ~0.4 mL PTFE test mass (Table of Materials) in air at T= 298 K by placing it on the pan of a calibrated analytical microbalance.</li>
	<li>Determine the volume V(298 K) of the test mass at T = 298 K using a gas pycnometer, or by dimensional measurements using calipers. If using dimensional measurements, the test mass must have a simple, precise shape (no tapers or rounded corners) and the volume of the through-hole (for the suspending line) should be determined.</li>
	<li>Calculate the mass m of the test mass by correcting the measured apparent mass for the buoyant force exerted by air according to the following: 
	<img alt="Equation" src="/files/ftp_upload/55761/55761eq2.jpg" /> 
	where &#961;air = 1.23 g/L (~0.1% correction).</li>
	<li>Place the microbalance on a stable platform roughly 10 cm above the Dewar flask and verify its calibration. Suspend the test mass using a 2 mil (50 &#956;m) monofilament line strung from the hook on the underside of the microbalance (designed for hanging mass measurements) and through a hole in the test mass. Determine the apparent mass in air, and compare with the measurement in step 3.3, correcting as needed for the mass of the line.</li>
	<li>Determine the volume V(77 K) of the test mass at T = 77 K by measuring its apparent mass in pure liquid nitrogen, mapp in LN2. Lower the test mass into the liquid nitrogen within the copper chamber until it is fully submerged. When boiling has ceased, measure the apparent mass. 
	NOTE: If the liquid nitrogen in the copper chamber is quiescent and air currents between the microbalance and the liquid surface are minimal, this mass can be measured to greater than &#177; 0.0002 g accuracy.</li>
	<li>Estimate the buoyant force on the submerged portion of the line, and verify that it is small compared with measurement errors.</li>
	<li>Calculate the volume and density of the test mass at 77 K using its known mass m and the measured apparent mass in LN2 at 77 K, mapp in LN2, according to the following: 
	<img alt="Equation" src="/files/ftp_upload/55761/55761eq3.jpg" /> 
	where &#961;LN2((77 K)=0.807 g/mL.</li>
</ol>
4. Preparation of the Initial Liquid N2-Ar Mixture
<ol>
	<li>Flow Ar gas at a flow rate of ~2 L/min through a coiled tube to its outlet. Place the coiled tube on top of the upper struts that stabilize the position of the copper chamber, just above the liquid nitrogen level and below the top surface of the Dewar. Cold N2 and Ar gas will build up above the cryogenic liquids, and thermal conduction, convection, and radiation will cool the tubing and the Ar gas within.</li>
	<li>After allowing the coiled tube to cool for 5 min, place the outlet of the tube in the copper chamber, at least 10 cm below the surface of the liquid nitrogen. Then cover the Dewar with the annular lid and transparent sheet.</li>
	<li>Adjust the Ar flow rate until Ar bubbles rise from the tube outlet to the top surface of the liquid nitrogen. Then reduce the flow rate until the bubbles form at the outlet but dissolve or liquefy just before breaking the liquid nitrogen surface. 
	NOTE: As the Ar concentration in the copper chamber increases, periodically adjust the Ar flow rate as needed to maintain bubble formation. If the Ar flow rate is too low, the Ar may freeze inside the tube and block the flow.</li>
	<li>Add liquid nitrogen as needed to maintain its level in the surrounding Dewar. Remove ice as it accumulates on cold surfaces.</li>
	<li>Periodically mix the liquid by inserting a thin (e.g., 35 micron), circular sheet of copper foil attached to a thin insulating rod into the copper chamber, and slowly moving it up and down like a piston. This will reduce concentration gradients and the tendency for Ar to crystallize out of the solution.</li>
</ol>
5. Measuring and Adjusting the Density of the Initial N2-Ar Mixture
<ol>
	<li>Calculate the target density for the N2-Ar mixture by estimating the T =77 K density of the sample to be measured, e.g., measurements at higher concentrations of the sample&#39;s non-aqueous component.</li>
	<li>Attach the test mass using the monofilament line to the hook on the underside of the microbalance measuring pan, measure its apparent mass in air, and confirm agreement with the measurement in step 3.1.</li>
	<li>Mix the N2-Ar solution to eliminate concentration and density gradients as in step 4.5.</li>
	<li>Precool the test mass to T = 77 K by lowering it into the liquid nitrogen outside the copper chamber. Raise the test mass into the cold layer of nitrogen gas above the liquid nitrogen, wait for residual liquid nitrogen to evaporate off the test mass, and then lower the cold, dry test mass into the N2-Ar mixture until it is fully submerged and within 2 cm of the liquid surface.</li>
	<li>After any boiling and surface waves have disappeared, measure the apparent mass of the test mass. Calculate the solution density using its true mass m and T = 77 K volume V(77K) from step 3.7 according to the following: 
	<img alt="Equation" src="/files/ftp_upload/55761/55761eq4.jpg" /></li>
	<li>Increase the solution density by flowing additional Ar until the desired initial density is obtained. Sample drops that sink can easily be lost, so the initial density should be at least a few percent higher than the expected sample density. The sample will then float, making it easier to keep track of it, and the N2-Ar mixture density will then only need to be adjusted downward by adding liquid N2.</li>
	<li>Remove the Ar precooling coiled tube and allow to warm and dry before next use.</li>
</ol>
6. Cooling Drops of Sample Solution
<ol>
	<li>Immediately prior to drop dispensing and cooling, repeat step 1.5 to mix the nitrogen/argon cryogenic liquid. Be careful not to introduce any bubbles.</li>
	<li>Remove the sample tube&#39;s airtight cap. Using a clean 1 mL syringe, extract up to 1 mL of solution, and replace the cap. Attach a 27 to 33 G needle onto the syringe, and then push a small amount of sample through the needle to expel air and any residues from previous dispensing.</li>
	<li>Two methods may be used to project sample drops into the N2-Ar mixture.
	<ol>
		<li>For samples with large non-aqueous component concentrations (&#62;45% w/w) that can be vitrified with modest cooling rates, lightly press the syringe to displace a small (250 &#956;m to 1 mm) diameter, ~10 nL to 1 &#956;L drop that hangs from the needle tip by surface tension. Gently tap the needle to detach and project the drop toward the liquid N2-Ar mixture.</li>
		<li>For samples that require faster cooling for vitrification, place the outlet of a gas tube connected to a vacuum generator (supplied by lab compressed air) in the gas space above the liquid N2-Ar mixture, and gently suction away the cold gas layer that forms. This increases cooling rates for small samples11.
		<ol>
			<li>Carefully touch the needle tip to a 25-75 &#956;m thick clear polymer strip to dispense a small volume (&#60;10 nL, corresponding to a drop diameter &#60;200 &#956;m) of sample. 
			NOTE: To obtain the smallest, most nearly spherical, and most easily removed drops, soak the strip in a hydrophobic coating solution for 10 min and let dry prior to sample dispensing.</li>
			<li>Grab the strip using an attached plastic or wood rod, and manually plunge the rod plus the strip into the liquid N2-Ar mixture.</li>
			<li>Once the drop has solidified and boiling has ceased, grab the strip edge opposite the rod using tweezers. Flex the strip, keeping it immersed in the liquid N2-Ar, until the sample drop pops off and floats to the surface.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
7. Assessment of the State of the Sample
<ol>
	<li>Using a long working distance (5-10 cm) binocular microscope and bright, cool illumination from an LED or fiber optic illuminator, carefully examine the drop while keeping it immersed in the liquid N2-Ar. Vitrified drops should appear clear12,13. Reject drops that are hazy or cloudy (likely containing more than one phase) and/or that show optical imperfections including cracks (which may be associated with voids that change the average density). 
	NOTE: Painting the inside surface of the copper chamber to provide a black background can facilitate the identification of sample imperfections.</li>
</ol>
8. Determination of the Density of the Sample
<ol>
	<li>The dispensed drop will initially float, sink, or (rarely) be neutrally buoyant in the N2-Ar mixture. Floating drops may sometimes be held up by surface tension, tiny bubbles, or adhered ice particles. Inspect the entire drop surface with the microscope. Displace the drop downward from the liquid surface using a small (2-3 mm diameter, 10 cm long) precooled plastic or wood rod and observe its response.</li>
	<li>If the drop sinks, increase the density of the liquid N2-Ar using the procedure in step 4 until the drop becomes neutrally buoyant or floats.</li>
	<li>If the drop floats, decrease the density of the liquid N2-Ar by adding liquid nitrogen using a 1.8 mL cryovial. At large initial mixture densities (1.2-1.3 g/mL), adding N2 in 1 mL increments gives appreciable density changes, but this should be increased toward 5 mL at low densities (0.8-0.9 g/mL). Gently mix the N2-Ar (so as not to lose track of the sample drop) using the thin perforated copper foil sheet up-and-down in the copper chamber.</li>
	<li>After each N2 addition, use a small pre-cooled rod to gently displace the floating drop downward into the liquid, and observe its speed as it returns to the surface.</li>
	<li>When the solution density has been adjusted so that the drop appears to be neutrally buoyant or ascends very slowly (&#60;50 &#956;m/s will ensure ~0.1% or better density accuracy for drops with volumes down to 50 pL), measure the N2-Ar mixture density as described in step 5. Then add additional liquid N2 until the drop first begins to slowly sink, and measure the N2-Ar mixture density again. These two measurements will provide bounds on the drop density.</li>
</ol>

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The authors have nothing to disclose.

The present apparatus and methods, developed primarily by undergraduates with limited access to instrument-building tools and machinery, nevertheless delivers highly accurate density measurements for individual liquid drops as small as 50 pL. In the concentration range near and above 50% w/w, where small cooling rates are sufficient to obtain vitrified samples, the densities agree with those obtained in previous measurements on bulk samples. Extrapolations of the present densities to 0% concentration &#8211; pure water &...

The physical properties of water and aqueous mixtures in their various phases are of fundamental interest, and are important to in vivo and in vitro understanding of biological systems. In contemporary cryobiology and biological cryopreservation, the vitreous or amorphous phases of aqueous cryoprotectant mixtures are of particular interest1,2. Nucleation and growth of ice crystals can disrupt cells and tissues, and promote protein denaturation and aggregation, so cryopreservation protocols that vitrify the solvent have become increasingly popular. In biomolecular crystallography, the crystallization of solvent in the channels between biomolecules disrupts crystal lattices and degrades diffraction properties. Vitrification is achieved through a combination of rapid cooling, dehydration, and addition of cryoprotective solutes such as glycerol, ethylene glycol, polyethylene glycols (PEGs), alcohols, and salts.
Vitrification limits ice crystallization and growth, but does not eliminate all cooling-related sample damage. For example, crystal mosaicity (a measure of the distribution of crystal plane orientations) routinely increases by a factor of 10 to 100 when protein crystals are cooled into a vitrified state3, and post-thaw survival rates of vitrified sperm cells and oocytes vary widely.
One damage mechanism is differential contraction of solvent and surrounding material during cooling3,4,5. The equilibrium solvent and solute concentrations within a crystal, cell or tissue are temperature dependent, and the solvent plus the solute and surrounding material may contract by different amounts. Rapid cooling may prevent solvent and solute redistribution prior to vitrification, and differential contraction may lead to large, inhomogeneous, nonequilibrium stresses that cause sample damage.
Rational approaches to reducing cooling-induced damage could thus benefit from the knowledge of temperature-dependent densities of liquid and vitrified aqueous mixtures. At solute concentrations above 50% weight of solute to weight of solution (w/w), most aqueous cryoprotectant mixtures can be vitrified with modest cooling rates of 10 K/s or less, allowing production of and density measurements using large vitreous samples6. Density can then be determined using Archimedes&#39; principle, by measuring the apparent weight of the sample when suspended in a liquid cryogen like nitrogen. However, as solute concentration decreases, cooling rates required for vitrification increase rapidly: Cooling rates for aqueous glycerol mixtures increase from &#60;10 K/s at 50% weight of solute in g to volume of solution in mL (w/v) to &#62;1,000 K/s at 25% w/v7. Heat transfer becomes boundary-layer limited, so that achieving larger cooling rates requires smaller and smaller samples8.
Measurements of the density of pure vitreous water and ice have been achieved by depositing micrometer-diameter (femtoliter volume) drops in a vacuum onto a cryogenically-cooled surface so as to build up a macroscopic (gram mass) sample. The density of this sample was determined by cryoflotation in a liquid nitrogen-argon mixture, in which the density of the cryogenic liquid was adjusted until the sample became neutrally buoyant9. However, generating large samples from a large number of small drops in a way that minimizes void volumes &#8211; an important source of error in previous vitreous phase density measurements &#8211; is non-trivial. For aqueous mixtures, differential evaporation of solution components during aerosolization and deposition in a vacuum can lead to substantial uncertainties in deposited concentrations.
We have developed a method, based on cryoflotation, that allows accurate density determination of aqueous mixtures using individual drops as small as 50 pL10. These drops can be rapidly cooled while retaining their original concentrations, and their cryogenic temperature state (vitrified or crystalline) can be assessed using a simple visual assay that correlates with X-ray diffraction measurements. This method is broadly applicable to aqueous and non-aqueous mixtures, and can be extended to a variety of biological samples including cells (e.g., stem and egg), tissue samples, and protein crystals having low-temperature densities between 0.8 and 1.4 g/mL.

<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">centrifuge tube</td>
			<td style="width:71px;">Falcon</td>
			<td style="width:119px;">6029236</td>
			<td style="width:167px;">15 mL conical centrifuge tube</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">glycerol, &#62;99.5%</td>
			<td style="width:71px;">Sigma&#160;</td>
			<td style="width:119px;">G9012-100 mL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">ethylene glycol, &#62;99.8%</td>
			<td style="width:71px;">Sigma</td>
			<td>324558-100 mL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">analytical microbalance</td>
			<td style="width:71px;">Mettler</td>
			<td style="width:119px;">AE240</td>
			<td>Analytical balance, 0.01 mg resolution, has hook on bottom for weighing below the balance&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">vortexer</td>
			<td style="width:71px;">Scientific Industries</td>
			<td style="width:119px;">SI-0236</td>
			<td>Vortex-Genie 2</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Apparatus enclosure framing</td>
			<td style="width:71px;">Unistrut&#160;</td>
			<td style="width:119px;">1-5/8&#34; metal framing</td>
			<td>48&#34; wide x 24&#34; deep x 40&#34; tall&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Apparatus enclosure air barrier</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>any clear plastic sheeting</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">neoprene rubber disk</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>4&#34; diameter, 1/8&#34; thick</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">dewar flask</td>
			<td style="width:71px;">Scilogix Dilvac</td>
			<td style="width:119px;">SS333</td>
			<td>4.5 liter dewar flask with steel case and clamp lid</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">copper chamber</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>This fabricated part is comprised of a 1.43&#34; diameter, 0.017&#34; wall thickness copper tube with a solid cylindrical copper base soldered to seal one end.&#160; The copper base is 0.87&#34; tall and the overall chamber height is 7&#34;.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">nitrogen gas</td>
			<td style="width:71px;">Airgas</td>
			<td style="width:119px;">NI HP300</td>
			<td>99.998% pure N2 gas</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">argon gas</td>
			<td style="width:71px;">Airgas</td>
			<td style="width:119px;">AR HP300</td>
			<td>99.998% pure Ar gas</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">rotameter</td>
			<td style="width:71px;">Omega</td>
			<td style="width:119px;">FL3692ST</td>
			<td>2.52 l/min max flow rate</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:216px;">foam insulating lid</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>This part is fabricated from 4 lb/ft3 crosslinked polyethylene foam (supplied by Technifab, 1355 Chester Industrial Parkway, Avon, OH), and has an OD of 2.42&#34;, and ID of 1.52&#34;, and a thickness of 0.79&#34;.&#160;&#160;&#160;&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PTFE test mass</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>This fabricated part is a 0.246&#34; diameter, 0.580&#34; tall cylinder with a 0.060&#34; diameter hole running perpendicular to and intersecting the cylinder axis ~0.10&#34; from one end.&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">microbalance platform</td>
			<td style="width:71px;">Unistrut</td>
			<td style="width:119px;">1-5/8&#34; metal framing</td>
			<td>11&#34; wide x 24&#34; long x 24&#34; high rectangular frame with an top aluminum sheet containing a hole for the monofilament and hanging test mass</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">2 mil (50 um) monofilament line</td>
			<td style="width:71px;">Berkley</td>
			<td style="width:119px;">NF1502-CM</td>
			<td>Nanofil fishing line</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Argon precooling coil tubing</td>
			<td style="width:71px;">VWR</td>
			<td style="width:119px;">60985-512</td>
			<td>1/8&#34; ID x 1/4&#34; OD PVC tubing</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:216px;">perforated copper foil mixer</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>1.4&#34; diameter,&#160; 35 micron thick copper disk, cut from 1 ounce/ft2 copper sheet and perforated with holes using an awl or other sharp pointed tool.&#160; Insert 1-2 mm diameter rigid thermally insulating (plastic or wood) rod into the center and fix using epoxy as needed.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">syringe</td>
			<td style="width:71px;">BD</td>
			<td style="width:119px;">309628</td>
			<td>1 ml Luer-Lok tip syringe</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">vacuum generator</td>
			<td style="width:71px;">Gast</td>
			<td style="width:119px;">VG-015-00-00</td>
			<td>compressed air venturi single stage vacuum generator</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">hydrophobic coating spray</td>
			<td style="width:71px;">RainX</td>
			<td style="width:119px;">620036</td>
			<td>plastic water repellent</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">long focal length stereo microscope</td>
			<td style="width:71px;">Bausch and Lomb Stereozoom 6</td>
			<td style="width:119px;"></td>
			<td>0.67-4 x zoom pod with 20x eyepieces, 10 cm working distance&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">LED ring illuminator</td>
			<td style="width:71px;">Amscope</td>
			<td style="width:119px;">LED144S</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">LED spot illuminator</td>
			<td style="width:71px;">Newhouse Lighting</td>
			<td style="width:119px;">NHCLP-LED</td>
			<td>3W LED gooseneck clamp lamp</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">1.8 ml cryo vial</td>
			<td style="width:71px;">Nunc</td>
			<td>V7634-500EA</td>
			<td>Any 1.8 or 2 ml cryovial is adequate</td>
		</tr>
	</tbody>
</table>

measuring the densities of aqueous glasses at cryogenic temperatures

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid cooling, to prepare the desired cryogenic temperature phase. Microliter to picoliter size drops are cooled by projection into a liquid nitrogen-argon (N2-Ar) mixture. The cryogenic temperature phase of the drop is evaluated using a visual assay that correlates with X-ray diffraction measurements. The density of the liquid N2-Ar mixture is adjusted by adding N2 or Ar until the drop becomes neutrally buoyant. The density of this mixture and thus of the drop is determined using a test mass and Archimedes principle. With appropriate care in drop preparation, management of gas above the liquid cryogen mixture to minimize icing, and regular mixing of the cryogenic mixture to prevent density stratification and phase separation, densities accurate to &#60;0.5% of drops as small as 50 pL can readily be determined. Measurements on aqueous cryoprotectant mixtures provide insight into cryoprotectant action, and provide quantitative data to facilitate thermal contraction matching in biological cryopreservation.

Density measurements at T = 77 K for vitrified drops of aqueous glycerol and ethylene glycol versus cryoprotectant concentration are shown in Figure 1A and Figure 1B respectively, and the corresponding change in specific volume between T = 298 K and 77 K, calculated using previously determined T = 298 K densities, is shown in Figure 2. At h...

Watch this Scientific Journal Video about Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures at JoVE.com

Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures

A protocol for determining the vitreous phase densities of micro- to pico-liter size drops of aqueous mixtures at cryogenic temperatures is described.

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid ...

measuring-the-densities-of-aqueous-glasses-at-cryogenic-temperatures

Research

JoVE Journal

Engineering

8.6K Views.  Cornell University. A protocol for determining the vitreous phase densities of micro- to pico-liter size drops of aqueous mixtures at cryogenic temperatures is described. 

Video: Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

Synthesis and Microdiffraction at Extreme Pressures and Temperatures

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary interiors, design materials with novel properties, understand the mechanical behavior of materials exposed to very high stresses as in explosions or impacts. Synthesis and structural analysis of materials at extreme conditions of pressure and temperature entails remarkable technical challenges. In the laser heated diamond anvil cell (LH-DAC), very high pressure is generated between the tips of two opposing diamond anvils forced against each other; focused infrared laser beams, shined through the diamonds, allow to reach very high temperatures on samples absorbing the laser radiation. When the LH-DAC is installed in a synchrotron beamline that provides extremely brilliant x-ray radiation, the structure of materials under extreme conditions can be probed in situ. LH-DAC samples, although very small, can show highly variable grain size, phase and chemical composition. In order to obtain the high resolution structural analysis and the most comprehensive characterization of a sample, we collect diffraction data in 2D grids and combine powder, single crystal and multigrain diffraction techniques. Representative results obtained in the synthesis of a new iron oxide, Fe4O5 1 will be shown.

The laser heated diamond anvil cell combined with synchrotron micro-diffraction techniques allows researchers to explore the nature and properties of new phases of matter at extreme pressure and temperature (PT) conditions. Heterogeneous samples can be characterized in situ under high pressure by 2D mapping and combined powder, single-crystal and multigrain diffraction approaches.

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary ...

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids&#160;and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.
The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.1&#160;Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic ...

Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

A Simple Dewar/Cryostat for Thermally Equilibrating Samples at Known Temperatures for Accurate Cryogenic Luminescence Measurements

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and a CCD spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, fast sample freezing, fast alignment of the sample, accurate and stable sample temperatures, and small size and portability of the Dewar/cryostat cryogenic unit. When coupled with the fast data acquisition rates of the CCD spectrograph, this Dewar/cryostat is capable of supporting cryogenic luminescence spectroscopic measurements on luminescent samples at a series of known, stable temperatures in the 77-300 K range. A temperature-dependent study of the oxygen quenching of luminescence in a rhodium(III) transition metal complex is presented as an example of the type of investigation possible with this Dewar/cryostat. In the context of this apparatus, a stable temperature for cryogenic spectroscopy means a luminescent sample that is thermally equilibrated with either liquid nitrogen or gaseous nitrogen at a known measureable temperature that does not vary (&#916;T &#60; 0.1 K) during the short time scale (~1-10 sec) of the spectroscopic measurement by the CCD. The Dewar/cryostat works by taking advantage of the positive thermal gradient dT/dh that develops above liquid nitrogen level in the Dewar where h is the height of the sample above the liquid nitrogen level. The slow evaporation of the liquid nitrogen results in a slow increase in h over several hours and a consequent slow increase in the sample temperature T over this time period. A quickly acquired luminescence spectrum effectively catches the sample at a constant, thermally equilibrated temperature.

A simple liquid nitrogen Dewar/cryostat apparatus comprised of a small fused silica optical Dewar, a thermocouple, and a charge-coupled device (CCD) spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, freezing, and alignment, accurate and stable sample temperatures, and small size/portability.

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and ...

Design and Evaluation of Smart Glasses for Food Intake and Physical Activity Classification

This study presents a series of protocols of designing and manufacturing a glasses-type wearable device that detects the patterns of temporalis muscle activities during food intake and other physical activities. We fabricated a 3D-printed frame of the glasses and a load cell-integrated printed circuit board (PCB) module inserted in both hinges of the frame. The module was used to acquire the force signals, and transmit them wirelessly. These procedures provide the system with higher mobility, which can be evaluated in practical wearing conditions such as walking and waggling. A performance of the classification is also evaluated by distinguishing the patterns of food intake from those physical activities. A series of algorithms were used to preprocess the signals, generate feature vectors, and recognize the patterns of several featured activities (chewing and winking), and other physical activities (sedentary rest, talking, and walking). The results showed that the average F1 score of the classification among the featured activities was 91.4%. We believe this approach can be potentially useful for automatic and objective monitoring of ingestive behaviors with higher accuracy as practical means to treat ingestive problems.

This study presents a protocol of designing and manufacturing a glasses-type wearable device that detects the patterns of food intake and other featured physical activities using load cells inserted in both hinges of the glasses.

This study presents a series of protocols of designing and manufacturing a glasses-type wearable device that detects the patterns of temporalis muscle ...

Processing of Bulk Nanocrystalline Metals at the US Army Research Laboratory

Given their potential for significant property improvements relative to their large grained counterparts, much work has been devoted to the continued development of nanocrystalline metals. Despite these efforts, the transition of these materials from the lab bench to actual applications has been blocked by the inability to produce large scale parts that retain the desired nanocrystalline microstructures. Following the development of a method proven to stabilize the nanosized grain structure to temperatures approaching that of the melting point for the given metal, the US Army Research Laboratory (ARL) has progressed to the next stage in the development of these materials - namely the production of large scale parts suitable for testing and evaluation in a range of relevant test environments. This report provides a broad overview of the ongoing efforts in the processing, characterization, and consolidation of these materials at ARL. In particular, focus is placed on the methodology used for producing the nanocrystalline metal powders, in both small and large-scale amounts, that are at the center of ongoing research efforts.

This paper provides a brief overview of the ongoing efforts at the Army Research Laboratory on the processing of bulk nanocrystalline metals with an emphasis on the methodologies used for the production of the novel metal powders.

Given their potential for significant property improvements relative to their large grained counterparts, much work has been devoted to the continued ...

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic glasses (MGs). Despite their amorphous nature, the identification of hyperfine interactions unveils faint structural modifications. For this purpose, we have employed two techniques that utilize nuclear resonance among nuclear levels of a stable 57Fe isotope, namely M&#246;ssbauer spectrometry and nuclear forward scattering (NFS) of synchrotron radiation. The effects of heat treatment upon (Fe2.85Co1)77Mo8Cu1B14 MG are discussed using the results of ex situ and in situ experiments, respectively. As both methods are sensitive to hyperfine interactions, information on structural arrangement as well as on magnetic microstructure is readily available. M&#246;ssbauer spectrometry performed ex situ describes how the structural arrangement and magnetic microstructure appears at room temperature after the annealing under certain conditions (temperature, time), and thus this technique inspects steady states. On the other hand, NFS data are recorded in situ during dynamically changing temperature and NFS examines transient states. The use of both techniques provides complementary information. In general, they can be applied to any suitable system in which it is important to know its steady state but also transient states.

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Here, we present a protocol to describe ex situ and in situ investigations of structural transformations in metallic glasses. We employed nuclear-based analytical methods which inspect hyperfine interactions. We demonstrate the applicability of M&#246;ssbauer spectrometry and nuclear forward scattering of synchrotron radiation during temperature-driven experiments.

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic ...

Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed electric body force has to be induced by ionic motions in liquid. On the other hand, positive and negative charges attract each other, and electroneutrality is maintained everywhere in equilibrium conditions. Furthermore, an increase in an applied voltage has to be suppressed to avoid water electrolysis, which causes the solutions to become unstable. Usually, EHD flows can be induced in non-aqueous solutions by applying extremely high voltages, such as tens of kV, to inject electrical charges. In this study, two methods are introduced to generate EHD flows induced by electrical charge separations in aqueous solutions, where two liquid phases are separated by an ion-exchange membrane. Due to a difference in the ionic mobility in the membrane, ion concentration polarization is induced between both sides of the membrane. In this study, we demonstrate two methods. (i) The relaxation of ion concentration gradients occurs via a flow channel that penetrates an ion-exchange membrane, where the transport of the slower species in the membrane selectively becomes dominant in the flow channel. This is a driving force to generate an EHD flow in the liquid. (ii) A long waiting time for the diffusion of ions passing through the ion-exchange membrane enables the generation of an ion-dragged flow by externally applying an electric field. Ions concentrated in a flow channel of a 1 x 1 mm2 cross-section determine the direction of the liquid flow, corresponding to the electrophoretic transport pathways. In both methods, the electric voltage difference required for an EHD flow generation is drastically reduced to near 2 V by rectifying the ion transport pathways.

The rectification of ion transport pathways is an effective method to generate one-directional ion-dragged electrohydrodynamic flows. By setting an ion-exchange membrane in a flow channel, an electrically polarized condition is generated and causes a liquid flow to be driven when an electric field is externally applied.

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed ...

Measurement of Dynamic Force Acted on Water Strider Leg Jumping Upward by the PVDF Film Sensor

This study aimed to make an explanation for the phenomenon in nature that water strider usually jumps or glides on the water surface easily but quickly, with its peak locomotion speed reaching 150 cm/s. First of all, we observed the microstructure and hierarchy of water strider legs using the scanning electron microscope. On the basis of the observed morphology of the legs, a theoretical model of the detachment from water surface was established, which explained water striders' capability to slide on water surface effortlessly in terms of energy reduction. Secondly, a dynamic force measurement system was devised using the PVDF film sensor with excellent sensitivity, which could detect the whole interaction process. Subsequently, a single leg in contact with water was pulled upward at different speeds, and the adhesion force was measured at the same time. The results of the departing experiment suggested a deep understanding of the fast jumping of water striders.

The protocol here is dedicated to investigating the free and quick maneuvering of water strider on water surface. The protocol includes observing the microstructure of legs and measuring the adhesion force when departing from water surface at different speeds.

This study aimed to make an explanation for the phenomenon in nature that water strider usually jumps or glides on the water surface easily but quickly, ...