Triggering salt crystal formation on demand#

Experiments in triggering sodium acetate trihydrate (SAT) crystal formation (nucleation)#

Will creating movement in SAT solution with flip dot actuator start nucleation?#

Before doing any thorough research, one initial hypothesis for why crystal formation was occurring was due to pressure changes in the solution as a result of the disc being flexed. Thus, I pursued creating a flip dot actuator in the sodium acetate solution to induce SAT crystal formation. I also wondered if a magnet could serve as a nucleating site (as the metal disc does in hand warmers) and so below is schematic of what I've tried:

The following subsections are some of my findings.

Performing flip dot experiments in open air environment will introduce impurities, cause spontaneous crystal formation#

I made a 2:1 sodium acetate:water solution. I put it in a petri dish, and stuck a magnetic coil underneath (45 loops around a pencil). I suspended a hematite magnetic bead by thread and submerged it into the salt solution. The bead was placed this in line with the magnetic coil.

Above: Aerial view of set-up, if you can see past the crystals

The magnetic wire was stripped of its coating with sandpaper so that I could attach a 9V battery to it and run current through it. Unfortunately, the salt formation was not triggered this way.

The magnet actuator seemed to work and disrupt the salt solution (as one can see from video above), but it did not induce salt formation. The salt formation seen in the figures above was induced by cooling of the top layer of liquid to air.

Some things to try for next time:

Magnetic movement will not induce crystal formation (but internal rough edges of sealed plastic bags might)#

Can we flex the nucleating disc with a solenoid?#

I tossed the flip dot triggering idea aside, and decided to hack existing products as listed on my research page. After all, the products have been out on the market for a while, so why not build off what's been done and works? I wondered if there was a way to flex the disc in the hand warmer bags with a push/pull solenoid. I envisioned securing the flex disc into the packaging of the hand warmer, and then securing a solenoid on the outside of the packaging to separate the electronics (the solenoid) from the flex disc in the SAT solution, just like the schematic below:

Turns out one needs a huge amount of force (160g) to flex the disc.

Small solenoid push/pull trials - the disc does not flex#

Even though (according to my calculations) the force (160g) required for disc flexing was higher than what the small push solenoid could provide (40g), I still had to validate (and optimistically hope) that the small push solenoid could flex my disc. I designed a small acrylic square ring that fit tightly around my push solenoid so that the majority of the force coming from the solenoid was purely coming from solenoid actuation. The result? The small push solenoid could not flex the disc, and if you don't believe me, watch the following video.

Large solenoid push/pull trials - the disc does not flex#

Similar to the small solenoid push/pull trials, I designed a holder for the flex disc so that the shaft of the large solenoid would be the only one trying to flex the disc. The following pictures show the result. I used an egg carton (holder for one egg), cut it so it was the exact height of the exposed shaft of the solenoid, and used hot glue to secure the outsides of the flex disc as well as to fill the inside of the egg carton holder to give it some weight.

I connected this holder to my larger solenoid which was further a power shield I already had on hand from SparkFun (which was connected to a larger computer power supply) and an Arduino which had the following code:

int solenoidPin = 6;    //This is the output pin on the Arduino we are using

void setup() {
  // put your setup code here, to run once:
  pinMode(solenoidPin, OUTPUT);           //Sets the pin as an output

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(solenoidPin, HIGH);    //Switch Solenoid ON
  delay(1000);                      //Wait 1 Second
  digitalWrite(solenoidPin, LOW);     //Switch Solenoid OFF
  delay(1000);                      //Wait 1 Second

My set-up for the solenoid connection to the Ardunio was:

The power shield had the resistor and MOSFETs built into it, so I just made sure the connection was consistent with the schematic.

Can we ‘grow’ SAT crystals using textiles?#

Based on findings gleaned from growing SAT crystals for the Textile as a Scaffold week, and the need to complete my Wearables II assignment, I wondered if I could create an actuator that prompted the growth of crystals -- essentially, half of the goal of my final project. I used the textile that yielded the most dramatic crystal growth, attached a vibrating motor to the other end of it, dipped the textile end into a 2:1 SAT:water solution, attached a 9V battery to the motor, and some crystals seemed to grow in solution as a result of the vibrating textile.

Actuator development#

Building off the finding that vibrating a solution of SAT in water could make crystals grow, I decided to make an actuator with textile, silicone, and a vibrating motor. See next section for process steps.

Process of making the actuator#


In case you needed it in a more detailed form, the steps are as follows:

  1. Make your mold out of laser cut acrylic (just like you were making an inflatable for soft robotics, but the 'inflatable' in this case would be an area that the vibration motor can nestle in)
  2. Pour silicone into mold, degas and cure in oven for 5 min at 150 deg F
  3. Take mold with silicone out of the oven, place textile square on top, apply pressure to make sure textile is adhered to silicone, and then stick assembly into oven @ 150F for another 5 min
  4. Once silicone is cured, take assembly out of the oven, remove silicone+textile out of the mold
  5. Place vibration motor into molded cavity
  6. Seal vibration motor in place by pouring silicone on top of vibration motor + silicone
  7. Soak entire assembly into melted sodium acetate trihydrate crystals, ensuring that the textile side coming into the most contact with the salts

Results - the actuator does not grow crystals on the textile#

To test the actuator, I stuck it in a 2:1 SAT:water solution I prepared previously. With the wires of the vibration motor hanging out,, I covered the solution with plastic saran wrap and also stuck a thermometer to monitor the solution temperature to know what was going on. I made sure to operate below the melting point of SAT. I connected a 9V battery to the vibration motor. If you watch the video below, you’ll see that the vibration doesn’t induce crystallization, especially in this closed environment. Secondly, because instantaneous crystallization didn’t happen and I kept the vibration motor running, the actuator start heating up the solution because the motor is working itself (you’ll see the temperature start to creep up).