Liquid Classes

Accounting for properties of liquids in your protocol can increase pipetting accuracy on the Flex. For example, a slower flow rate can improve pipetting for a viscous liquid, and an air gap can prevent a volatile liquid from dripping onto the deck.

This page covers the properties of Opentrons-verified liquid classes and how to use them in your protocols.

Opentrons-verified Liquid Classes

Opentrons-verified liquid classes are based on the properties of common liquids: water, ethanol, and glycerol.

Opentrons-verified liquid class	Description	Load name
Aqueous	Based on deionized water	water
Volatile	Based on 80% ethanol	ethanol_80
Viscous	Based on 50% glycerol	glycerol_50

Use an Opentrons-verified liquid class in your transfers to automatically apply optimized behavior. For example, choosing the glycerol_50 liquid class changes properties, like flow rate, to accurately transfer viscous liquid.

Liquid Class Properties

When you select a liquid class to use in transfers on the the Flex, properties like submerge speed, flow rate, touch tip, and air gap are automatically applied. These changes might help prevent splashing or dripping of a volatile liquid, or reduce air bubbles forming in a viscous liquid.

Each Opentrons-verified liquid class is defined by a set of properties:

Property	Description
Submerge position	The pipette begins at this position above the liquid.
Submerge speed	The pipette submerges into the liquid at this speed.
Delay after submerging	The pipette delays a specified amount of time: before submerging into or retracting from liquid. before or after an aspirate or dispense. after a push out.
Mix liquid	The pipette mixes liquid inside the well before an aspirate or after a dispense.
Pre-wet tip	The pipette pre-wets the attached tip before aspirating liquid.
Aspirate flow rate	The pipette aspirates liquid at this speed. Varies by volume.
Dispense flow rate	The pipette dispenses liquid at this speed. Varies by volume.
Retract position	The pipette retracts from the liquid and moves to this position.
Retract speed	The pipette retracts from the liquid at the specified speed.
Push out	The pipette dispenses a small amount of air to ensure all liquid leaves the tip. Varies by volume.
Touch tip	The pipette touches the attached tip to the sides of a well to remove droplets.
Air gap	The pipette aspirates a small amount of air after an aspirate or dispense. Varies by volume.
Blow out	The pipette dispenses a larger amount of air to ensure all liquid leaves the tip.

A liquid class definition specifies values for each property. When your Flex protocol includes a liquid class, these property values automatically define transfer behavior. For example, if you use .transfer_with_liquid_class to transfer a viscous liquid, the pipette submerges into the liquid and aspirates more slowly to prevent air bubbles from forming.

Using Liquid Classes

You’ll use a liquid class definition in your protocol to optimize transfer behavior based on liquid properties, along with your chosen Flex pipettes and tips.

This section covers selecting a liquid class and using the transfer_with_liquid_class() method. For more details, including using the distribute_with_liquid_class() and consolidate_with_liquid_class() methods, see Complex Commands.

Start by definining the tips, trash, pipette, and labware used in your transfers. Then, use ProtocolContext.get_liquid_class() to select an Opentrons-verified liquid class and save its results to a variable. get_liquid_class() takes into account the pipette and tip racks in your protocol and only loads the relevant portion of the liquid class definition.

from opentrons import protocol_api

requirements = {"robotType": "Flex", "apiLevel": "2.24"}

# define tips, trash, and pipette
def run(protocol: protocol_api.ProtocolContext):
    tiprack = protocol.load_labware(
        load_name="opentrons_flex_96_tiprack_50ul", location="D3"
    )
    trash = protocol.load_trash_bin(location="A3")
    pipette = protocol.load_instrument(
        instrument_name="flex_1channel_50",
        mount="left",
        tip_racks=[tiprack],
    )

# load source and destination labware
    reservoir = protocol.load_labware(
       load_name="nest_12_reservoir_15ml", location="C3"
    )
    plate = protocol.load_labware(
        load_name="nest_96_wellplate_200ul_flat", location="C2"
    )

# select liquid class to use in your protocol
    viscous_liquid = protocol.get_liquid_class(name="glycerol_50")

New in version 2.24.

Next, use the InstrumentContext.transfer_with_liquid_class() method to transfer an aqueous, volatile, or viscous liquid defined in a Flex protocol. This method requires the stored set of properties defined earlier, viscous_liquid, instead of the glycerol_50 load name. It accepts additional arguments that let you specify your liquid, volume, source and destination wells, tip handling preferences, and trash location.

Opentrons-verified liquid class definitions are based on Flex pipette and tip combinations. The API will raise an error if you try to perform a liquid class transfer with an OT-2 pipette and tips.

In the example below, a Flex P50 1-channel pipette will transfer 50 µL of your viscous_liquid from well A1 of the reservoir to well A1 of the destination plate. A new tip is used for each well transfer, and each tip is dropped in the trash bin loaded in slot A3.

# transfer with the viscous liquid class
pipette.transfer_with_liquid_class(
   liquid_class=viscous_liquid,
   volume=50,
   source=reservoir["A1"],
   dest=plate["A1"],
   new_tip="always",
   trash_location=trash,
)

New in version 2.24.

Here, the glycerol_50 viscous liquid class definition accounts for all other transfer behavior, like flow rate, whether or not to add an air gap or delay, and submerge and retract speeds. For each aspirate, the pipette:

• Moves to 2 mm above the top of the source well at 4 mm/sec.

• Submerges to 2 mm above the bottom of the source well at 4mm/sec.

• Aspirates 50 µL at 50 µL/sec with a correction of -0.2 µL.

• Delays for 1 second.

• Retracts to 2 mm above the top of the well at 4 mm/sec.

And for each dispense, the pipette:

• Moves to 2 mm above the top of the destination well at 4 mm/sec.

• Submerges to 2 mm above the top of the destination well at 4 mm/sec.

• Dispenses 50 µL at 25 µL/sec with a correction of -0.2 µL.

• Pushes out a volume of air equivalent to 3.9 µL

• Delays for 0.5 second.

• Retracts to 2 mm above the top of the well at 4 mm/sec.

In many cases, the liquid class definition represents fine-tuned changes optimized for each liquid class. If you instead use the Flex P50 1-channel pipette to transfer 50 µL of the volatile liquid_2, transfer behavior would include:

• Submerging into and retracting from the volatile liquid_2 at 100 mm/sec.

• Adding larger air gaps after aspirating and dispensing to prevent dripping onto the deck.

• Aspirating and dispensing at 30 µL/sec with a larger correction by volume.

• Pushing out a larger volume of air to ensure all liquid leaves the tip.

Not all transfer behavior is easily visible. See Liquid Class Definitions for a full list of changes based on liquid class, pipette, and tip combination. For more detail on individual transfer settings, see Liquid Control.