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.
High Viscosity Liquids
Low Viscosity Liquids
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
Low Viscosity Liquid
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.
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.
The density of different liquids and solids is shown below to visualize the range (in g/cm3):
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
Low Adhesion Liquid
High Cohesion Liquid
Low Cohesion Liquid
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 Mercury
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
Low Capillary Action Liquid
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.
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
Low Surface Tension Liquid
Low Contact Angle
High Contact Angle
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
Low Contact Angle
High Vapor Pressure
Low Vapor Pressure
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.
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
Low Vapor Pressure Liquid
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