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Successful Liquid Transfers Require Understanding Liquid Properties

liquid properties in automated liquid handling

Liquids tested in labs are as varied as the industries they appear in—everything from sticky honey to a fast-flowing petroleum. Depending on what the lab needs to test, these varied liquid types can be transferred by either hand pipettors or Hamilton automated liquid handlers.

Successful liquid transfers involve knowing the type of liquid and accounting for its behavior in a particular environment. A lab tech can rely on experience and intuition to inform how each liquid should be treated. The lab tech learns where to position the tip to make sure there are no bubbles as the liquid dispenses, where to hold the tip to prevent contamination between samples, and how to position the tip and for how long to make sure the liquid dispenses completely.

While humans can readily assess the liquid’s attributes and adapt accordingly, an automated liquid handling platform must know everything about the liquid in advance. The properties in this section of our Automated Liquid Handling Guide all have an impact on the liquid handling information given to the automated liquid handler, which makes all the difference in a successful transfer.

All liquid properties are affected to some degree by environmental conditions such as temperature, atmospheric pressure, humidity, etc. These conditions along with their influences are described in the sections below.

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What is Viscosity?

Viscosity describes the flow behavior of a liquid.

high viscosity liquids

High Viscosity Liquids

High viscosity liquids have a thicker flow and are less fluid. Examples of high viscosity liquids are honey and glycerin.
Low Viscosity Liquids

Low Viscosity Liquids

Low viscosity liquids have a thinner flow and are more fluid. Examples of low viscosity liquids are water and petroleum.

Why is Viscosity Important in Liquid Handling ?

Viscosity impacts the way a liquid is transferred in very specific ways. The spectrum below shows two example liquids and how to convey the property to the automated liquid handler.

High Viscosity Liquid

Use a lower flow rate. If you try to aspirate too quickly, suction in the tip prevents the fluid from entering the tip, resulting in inaccurate volumes. Use a surface dispense mode. Jet dispensing isn’t as effective with high viscosity liquids.

Low Viscosity Liquid

Use a higher flow rate. The goal is to get the liquid into the tip as fast and effectively as possible. Low viscosity liquids don’t resist entering the tip, so the flow rate can be higher.

Spectrum of Viscosity

The following chart shows a spectrum of viscosity for a variety of liquids. Note that many liquids such as water and ethanol have similar values for viscosity, but when compared to liquids such as glycerin and honey, the scale of the difference is stark.

Viscosity of sample liquids at 20C

What is Density?

Density is a measurement of how much of a substance can occupy a space. A material’s density gives the impression of heaviness or lightness. Density is affected by temperature and atmospheric pressure.

High density example

High Density

Low density example

Low Density

Why Is Density Value Important in Liquid Handling?

The density value for a liquid is required for measuring liquid transfers gravimetrically. The volumetric mass density of a substance is its mass per unit volume. If the weight of a liquid transfer is determined gravimetrically, then the transfer volume can be determined. Density is often provided on material safety data sheets, but can also be determined empirically.

Mixtures of liquids with different densities may not remain in suspension, which can make it hard to transfer a mixture properly. A suspension is a mixture of two or more substances that essentially become one homogenous liquid. However, some suspensions eventually settle and separate. Liquids with less density rise to the top, while the heavier density liquids fall to the bottom.

Many applications require the addition of one liquid to another with the eventual transfer of a single solution. Awareness of the density of different liquids can guide you to mix your samples before aspirating, or aspirating quickly before settling can occur.

Density Ranges

The density of different liquids and solids is shown below to visualize the range (in g/cm3):

density of various solids graph
density of various liquids graph

What are Adhesion and Cohesion?

Adhesion and cohesion are two different ways to describe the stickiness of a liquid. Liquids can be high in one or both with different impacts on the liquid transfer.

Adhesion

Adhesion

Adhesion is the measure of how much the liquid wants to stick to other substances. A high adhesive liquid like glue wants to bind to substances nearby, like tips and labware. A low adhesive liquid like mercury resists nearby substances.
Cohesion

Cohesion

Cohesion is the measure of how much the liquid wants to stick to itself. A high cohesive liquid like mercury wants to bind to itself and remain together. A low cohesive liquid like methane disperses and moves away from itself.

The adhesive and cohesive properties of a liquid are affected by the chemical compatibility of the tips and labware used to make a liquid transfer. Some liquids and materials are attracted to one another while others are inert when placed together. Chemical compatibility information is important to understanding how a liquid will interact with the various parts of the automated liquid handler and labware involved in the transfer.

The pressure of adhesion and cohesion results in capillary action, which causes the liquid to work against gravity. Capillary action occurs when the adhesion of the liquid to the tips or labware is stronger than the cohesive forces between the liquid molecules.


Why Are Adhesion and Cohesion Important in Liquid Handling ?

Adhesion and cohesion impact the way a liquid is transferred in very specific ways. The spectrum below shows two example liquids and how to convey the property to the automated liquid handler.

High Adhesion Liquid

Increase the blowout volume. More force is needed to get a sticky substance out of the tip.

Low Adhesion Liquid

Decrease the blowout volume. The liquid doesn’t resist being removed from the tip, so not as much force is needed to remove the liquid from tip.

High Cohesion Liquid

Decrease the air transport volume. Since the liquid wants to stick to itself, less force is needed to hold the liquid in the tip. May need to increase the blowout volume.

Low Cohesion Liquid

Increase the air transport volume to protect from drips. The increased air transport volume ensures that the liquid has more of an air buffer inside the tip during transport.

What is Capillary Action?

Capillary action is the tendency of a liquid to work against gravity and atmospheric pressure. Liquids with high capillary action can actually climb the tip in small volumes. Capillary action occurs when the adhesion and cohesion qualities of a liquid work together to move the liquid upward. Other properties can impact capillary action, including vapor pressure and environmental variables.

Liquids with low capillary action or any liquid in larger volumes generally are drawn downward by gravity and atmospheric pressure.

Capillary action of water

Capillary Action of Water

For water, the adhesion is greater than the cohesion, which results in a meniscus that turns downward and could cause the liquid to move up inside a tip.
Capillary action of mercury

Capillary Action of Mercury

Mercury has high cohesion and low adhesion. It results in a meniscus that turns downward and will not climb into the tip without help from the channel.

Why Is Capillary Action Important in Liquid Handling ?

Capillary action impacts the way a liquid is transferred in very specific ways. The spectrum below shows two example liquids and how to convey the property to the automated liquid handler.

High Capillary Action Liquid

Be careful with low volumes. Especially when working with 10 or 50 μL tips, capillary action can have a bigger effect. More liquid can be aspirated than intended.

Low Capillary Action Liquid

No changes to make. Since the liquid has no tendency to climb the tip, nothing extra needs to be taken into account for this factor.

What is Surface Tension?

Surface tension is the elastic tendency of liquids that makes the liquid acquire the least surface area possible. In terms of liquid handling, the surface tension affects how well the molecules at the surface of the liquid cohere to each other and to the walls of the tip.

The chart shows the surface tension of various liquids. The higher the number, the higher the surface tension and the more likely the liquid is to form a strong cohesion at the surface of the liquid.

Graph: Surface tension of various liquids

Why Is Surface Tension Important in Liquid Handling ?

Surface tension impacts the way a liquid is transferred in very specific ways. The spectrum below shows two example liquids and how to convey the property to the automated liquid handler.

High Surface Tension Liquid

Use a lower air transport volume. Liquids with high surface tension stay in the tip and don’t require a large air buffer to hold the liquid in. Use a high swap speed to break the connection between the liquid in the well or tube and the tip. Use side touch or minimize the distance from the end of the tip and the labware to allow the liquid to be more easily removed from the tip.

Low Surface Tension Liquid

Increase the air transport volume. Since the liquid wants to leave the tip, increasing the transport volume provides a greater air buffer in the tip. Decrease the settling time to minimize the amount of time the tip is in contact with the liquid and prevent time for droplet formation.

What is Contact Angle?

The contact angle is the angle the liquid makes to the surface of the labware.

Low contact angle

Low Contact Angle

A low contact angle like 0° makes it harder for low volumes to be transferred, since the liquid spreads out very thinly.
High contact angle

High Contact Angle

A high contact angle like 180° causes the liquid to stand taller in smaller volumes, but can make it difficult to aspirate and dispense since the cohesion can be repellent.

The contact angle is also affected by the type of surface where the liquid sits. See below for how different surfaces are characterized:

  • On a hydrophilic surface, the contact angle will be low (0°).
  • On a hydrophobic surface, the contact angle will be high (90°).
  • On a super hydrophobic surface, a contact angle will be higher than 160°.

The figure below shows a variety of contact angles, ranging from 0° to 180°.

Examples of Contact Angles

  • 100 μL water on hydrophobic coated plastic has a contact angle of ~130°.
  • 100 μL water on untreated plastic has a contact angle of ~90°.
  • Water on an aluminum surface has a contact angle of ~30°.
  • Ethanol on an aluminum surface has a contact angle lower than water.


Why Is Contact Angle Important in Liquid Handling ?

The contact angle impacts the way a liquid is transferred in very specific ways. The spectrum below shows two example liquids and how to convey the property to the automated liquid handler.

High Contact Angle

Make sure the tip enters the liquid properly. If the contact angle is high, the liquid might deform around the tip and cause issues with the volume.

Low Contact Angle

Be careful with low volumes. A liquid with a low contact angle may be hard to aspirate in low volumes because the height of the liquid is minimal. Use bottom touch off during aspiration and dispense to reach the very bottom of the well or tube. Consider using side touch during dispense.

What is Vapor Pressure?

Vapor pressure is the pressure created by vapor above a liquid’s surface. As atmospheric pressure pushes downward, the liquid’s vapor pressure is pushing upward.

High vapor pressure

High Vapor Pressure

Liquids with high vapor pressure off-gas since the atmospheric pressure isn’t enough to hold them down.
Low vapor pressure

Low Vapor Pressure

Liquids with low vapor pressure don’t off-gas or try to escape into the air.

Liquids with high vapor pressure also tend to evaporate quickly. A substance with a high vapor pressure at normal temperatures is referred to as volatile. Liquids with high vapor pressure include alcohols and ethers.

The chart below shows a temperature and pressure comparison for three liquids. Note that 760 mmHg is atmospheric pressure.

Environmental properties in the lab can have a large impact on vapor pressure. Changes in the elevation alter the atmospheric pressure, which can in turn change when the liquid begins to off-gas.

Additionally, if the temperature of the liquid increases, the vapor pressure also increases, just as water boils and turns to steam.

Graph: Vapor Pressure Curves

Why Is Vapor Pressure Important in Liquid Handling?

Vapor pressure impacts the way a liquid is transferred in very specific ways. The spectrum below shows two example liquids and how to convey the property to the automated liquid handler.

High Vapor Pressure Liquid

Use Anti-Droplet Control. Liquids with high vapor pressure are more likely to drip. Pre-wet the tip. The processing of pre-wetting involves aspirating and immediately dispensing some liquid to allow the tip’s vapor pressure to adjust, then aspirating enough volume for the liquid transfer. Use a larger blowout volume to allow room for the vapor.

Low Vapor Pressure Liquid

No changes to make. Since the liquid isn’t likely to drip based on off-gassing, nothing extra needs to be taken into account for this property.

What are Environmental Influences?

Environmental influences are properties of the space around the liquid that impact the behavior of the liquid itself.

Here are some of the important environmental influences to consider along with their effects on liquids:

Icons environmental humdity

Humidity

Low humidity results in a higher evaporation rate since there is less liquid in the air. There may also be higher instances of static electricity, which could impact low volume pipetting. High humidity results in a lower evaporation rate since the air is already saturated.
Icons environmental atmospheric

Atmospheric Pressure

A decrease in pressure increases the rate of evaporation. Atmospheric pressure changes with the elevation and is closely linked to vapor pressure. For example, if creating a liquid class at sea level, test the liquid class again before using it at a lab at 6,000 feet elevation.
Icons environmental radiation

Radiation

Changes the chemical properties of a substance.
Icons environmental temperature

Temperature

All liquid properties can shift with temperature, even when the change is just a few degrees: vapor pressure, atmospheric pressure, density, viscosity. It is recommended to control for temperature in the lab. Always develop liquid classes at common lab temperatures. Record the temperature during development.
Icons environmental vibrations

Vibrations

Vibrations can cause drops to form. Ideal to have no vibrations, especially during gravimetric measurements.

Why They're Important

The automated liquid handler can only execute a programmed sequence of activities for a set liquid class. Any changes in the environment that alter the liquid properties without the automated liquid handler knowing jeopardize the quality of the liquid transfer.

For example, if the temperature in a lab is usually controlled, but one day spikes due to an issue with the air conditioner, there could be a change in the liquid transfer due to the increased heat.

If the environmental conditions change, the performance of the liquid transfers must be confirmed again.

Additional Resources for Automated Liquid Handling

Learn about all of Hamilton Company's Automated Liquid Handling solutions
Check out the homepage of our Automated Liquid Handling Guide
Read our recommendations for Step by Step Automated Liquid Handler Setup
Get our input on the process used to Measure Transferred Liquid Volume
Want to "own" the guide? Click for a PDF Downloadable Liquid Handling Guide
Read our tips to accurately Pipette Volatile Liquids
Read our comparison of Manual Pipetting vs Semi Automation vs Automation
Hamilton Robotics highlights 10 Important Considerations for Accurate Automated Pipetting
Read our advice on Addressing Challenges of Automated Pipetting