What separates quartz from liquid crystal?

When you check your watch, you’re probably not considering the stark contrast between quartz and liquid crystals. And perhaps you shouldn’t – after all, analyzing their differences might make you late for your next engagement. Even though quartz and liquid crystals are in entirely different states of matter, they do share one crucial trait: your average digital watch or clock simply cannot function without them.

Before we dive into their roles inside a watch, let's explore the properties of each material. Quartz is one of Earth's most plentiful minerals. Known chemically as silicon dioxide, quartz is a crystalline solid used in a variety of applications, such as jewelry and sandpaper. It's also common in devices like cell phones, television receivers, and of course, timepieces. Quartz is widely used in electronics due to its piezoelectric properties, meaning it generates an electrical charge when pressure is applied. Quartz also exhibits a reverse piezoelectric effect: applying an electrical charge causes the crystal to vibrate. These properties are vital for accurate timekeeping.

On the other hand, liquid crystals are not a single substance. They were first discovered in the 19th century by scientists examining cholesterol’s molecular weight. Liquid crystals are actually a group of compounds that represent a fourth state of matter: Their rod- or plate-shaped molecules flow like liquids, but maintain the alignment and order characteristic of solids [source: Nobelprize.org]. Liquid crystal displays (LCDs) are a common feature in devices such as computers and televisions, and this is due to the optical properties of liquid crystal molecules, which affect light as it passes through them. The first watch with a liquid crystal display was released in 1973 [source: The New York Times].

Now that we have a grasp on the distinct properties of quartz and liquid crystals, let’s explore how each of them functions within a watch.

The Function of Quartz

Let’s revisit the fundamental components necessary for a watch or clock to operate properly:

Keep in mind, we are focusing on digital quartz watches and clocks. These differ from mechanical watches, which require winding and generally do not incorporate quartz, as well as from analog watches and clocks that use gears to move the hands across a face, which typically don’t include liquid crystals.

Quartz serves as the primary timekeeping mechanism. First introduced into watches by Seiko in 1969, quartz crystals have since become the standard for modern watchmakers [source: Seiko]. The quartz in watches vibrates at an exceptionally high frequency, resulting in impressive accuracy. A quartz watch’s timekeeping can deviate by only a few seconds each month [source: Lombardi]]. Furthermore, due to quartz's piezoelectric properties, it requires minimal power to vibrate, meaning a single alkaline battery can keep a quartz watch running for years.

Quartz operates in conjunction with the watch’s integrated circuit. In essence, the battery sends a small electric charge to the circuit, which is connected to a quartz oscillator via tiny electrodes. The electric charge is then passed through the oscillator, causing it to vibrate. Watches are standardized with quartz oscillators that vibrate 32,768 times per second, or 32.768 kilohertz. The integrated circuit counts these vibrations and calculates time intervals – seconds, minutes, hours, and beyond. At every interval, the circuit sends out an electric pulse.

So, where does this pulse go? To the liquid crystal display. Want to know how liquid crystals display the numbers on a digital watch face? Keep reading to find out.

The Function of Liquid Crystal

While quartz tracks time in imperceptible micro-moments, liquid crystal plays a key role in presenting time in a format that's easy for us to understand on the display.

As we discussed earlier, liquid crystals have optical properties that make them ideal for use in LCD screens. These substances fall along a spectrum between solid and liquid, depending on their specific traits. The most common type of display in digital watches is called a twisted nematic (TN) display. In this display, the liquid crystal molecules are arranged in a spiral twist. Applying an external force, like an electric pulse, can untwist the molecules, which then alters the light passing through them.

An LCD on a watch essentially sandwiches liquid crystal material between two electrically conductive glass plates, each equipped with polarization filters positioned 90 degrees apart. The liquid crystal molecules align with the polarization filter, letting ambient light reflect off a mirror placed beneath the filter, giving the LCD its shiny appearance. When an electric charge is applied, the liquid crystals shift to a perpendicular orientation, and the filter absorbs all light, leaving black spaces on the display.

When the integrated circuit sends out electric pulses marking each second, minute, hour, and other time intervals measured by the quartz oscillator, it triggers the LCD to activate and deactivate specific parts of its display. The LCD is divided into seven- or 14-segment sections, each displaying a numeral from zero to nine. By combining these sections, the display can show the progression of time, like from 12:15 to 12:16, or 1:00 to 2:00. (Due to having more cells, 14-segment sections can also display letters along with numbers.)

As demonstrated, quartz and liquid crystal are distinct substances with very different functions in timekeeping. However, without the cooperation of both, we'd likely find ourselves running behind schedule.