What better way is there to conduct an experiment in freefall than with a sounding balloon?
While much is known about the physical properties of fluids in normal earth gravity, much less is known about how gravity affects these properties, such as capillary waves. Normally, the primary variable that affects the properties of a capillary wave is the surface tension of the fluid as acceleration due to gravity is constant. In this experiment however, the fluid properties are held constant while the normal force is varied. Thus, the purpose of this experiment is to study the how gravity affects capillary waves.
In order to conduct the experiment, a sounding balloon carries an experiment payload to an altitude of 30 kilometers, allowing for extended freefall. This payload consists of an air-shell surrounding a pressurized capsule containing a surfactant-water solution as well as sensors, electronics, a vibration apparatus, and a radio transmitter to collect and record data and to transmit the payload’s position to facilitate recovery. The water’s surface tension is lowered with ammonium dodecyl sulfate, a surfactant, to lower the energy required to generate capillary waves. Once the balloon and payload reach altitude, the payload is programmed to jettison the balloon, releasing the air-shell into freefall, or a normal force of zero. This triggers a routine that vibrates the capsule containing the fluid, generating capillary waves whose properties are recorded using a capacitance sensor. As air density and the air-shell’s speed increase during the decent, the capsule’s rate of acceleration decreases, thus increasing the normal force as the experiment progresses and allowing gravity’s influence to be measured.
Materials and Schematics:
Flight Track #1:
Video of Launch #1:
A capillary wave is defined as any wave that moves along the boundary of a fluid. The two main forces that govern such waves are the surface tension of the fluid and the acceleration forces the medium experiences. In order to study the relationship between these two forces, one of the forces must be changed. Since surface tension is a fundamental property of any fluid being studied and thus cannot be changed, one must alter the acceleration the fluid experiences in order to study the relationship between the two forces. By allowing an object, in this case a capsule containing the fluid, to accelerate downward with gravity, one can minimize the effect of gravitational forces on said object.
While the surface tension of any given solution is a fundamental property of said solution, the choice and concentrations of such a solution can still be taken into account in order to have the desired surface tension. Surfactants, such as the ammonium dodecyl sulfate used in this experiment, are chemicals that change the surface tension of a solution and allow a desired surface tension to be reached. In addition to changing the surface tension of a solution, surfactants change the wetting properties of a solution as well. A surfactant is used in this experiment in order to increase the wetting properties of water so the solution will better stick to the sides of the capsule it is within, producing more accurate measurements than if some of the solution was free-floating.
An understanding of the relationship between the effects of surface tension and gravity on capillary waves can prove useful anywhere acceleration is not constant. In any situation where a liquid is accelerated, the properties of capillary waves in it change. This could lead to unexpected results if one does not understand the relationship between acceleration and surface tension on capillary waves, giving purpose to this experiment.
Tracking an object that reaches an altitude of 30km and has the potential to travel dozens of kilometers can be difficult due to regulations on transmission power in unlicensed radio spectrum. These power restrictions however are lifted in the amateur radio spectrum. In addition, the Automatic Packet Reporting System (APRS), a data transmission protocol along with ground receiving sites designed for this sort of tracking, already exists. By designing and constructing a transmitter for this system, position reports will be received by APRS Internet gateways that relay said position reports to the Internet. Software written to interact with this system can then relay these position reports to SMS for tracking a balloon flight with a cell phone. Conducting an experiment remotely at these distances also leads to difficulties. The electronics and software must be designed to detect when the desired altitude is reached or freefall is attained and automatically start and record data from the experiment. Once terminal velocity is reached and data collection stops, the electronics must then deploy a parachute to facilitate a safe landing.
Data collection during the experiment consisted of two insulated electrodes inserted into the capsule and connected to a capacitance sensor. Since water’s dielectric constant differs from that of air, the capacitance measured across the electrodes fluctuates with the fluid level in the capsule. Capacitance measurements are taken at regular intervals along with acceleration measurements so amplitude and frequency can be determined for a given normal force. This capacitance data, taken from periods of differing acceleration, was then processed with a Fourier transformation to determine the frequency and relative amplitudes of waves at said accelerations.
A review of the data in freefall shows high amplitude waves grouped at frequencies below six hertz, while data taken on the ground shows a pronounced frequency of approximately nine hertz as well as a greatly decreased amplitude. From this data one is able to deduce that increased acceleration forces on capillary waves dampen their amplitude. Changes in frequency however are not as certain, as while there is a definite increase in amplitude in freefall, the frequency data is not as defined. From the data available it can be deduced that the lowest frequency component was a result of the preliminary drop off of a bridge as the second or so of freefall did not allow enough time for the fluid to stabilize. If the lowest frequency component is eliminated, there appears to be a dominant oscillation frequency at five hertz. While the data recorded over an extend period of time from the drop from a balloon would greatly improve the accuracy of freefall frequency measurements, it can be deduced from the data available that increased acceleration forces on capillary waves increase their frequency.