By Donal O’Sullivan, BSc. Reviewed by Michael Anderson, MBA.

Viscous liquids move through pipette tips more slowly than typical aqueous samples, so routine pipetting technique may not provide consistent aspiration and dispensing. More deliberate liquid handling — including appropriate aspiration and dispense speed, sufficient time for liquid movement and, where suitable, reverse pipetting — can improve transfer consistency. There is no single technique that is best for every viscous liquid. The appropriate approach depends on the liquid, transfer volume, pipette system and required performance. Low-retention tips may help where liquid is also being retained on the tip surface, but they do not remove the effects of viscosity itself.

 

Key Takeaways

• Viscous liquids generally require slower, more deliberate aspiration and dispensing because they move through the pipette tip more slowly than typical aqueous liquids.
• Allowing sufficient time for liquid movement can help reduce incomplete aspiration or dispensing, but there is no universal dwell time or speed suitable for every viscous liquid.
• Reverse pipetting can be useful for some viscous-liquid transfers, but it should not automatically replace forward pipetting for every application.
• Positive-displacement pipettes may be worth considering where air-displacement pipetting does not provide suitable performance with a difficult liquid.
• Low-retention tips can help reduce liquid retained through interaction with the tip surface, but they do not eliminate viscosity-related flow behaviour.
• Where transfer performance matters, assess and, where practical, verify the pipette, tip, liquid, volume and technique together under representative conditions; gravimetric checks should use the liquid’s density where delivered volume is calculated from mass.

Why Are Viscous Liquids More Difficult to Pipette?

Most routine laboratory pipetting involves relatively free-flowing aqueous liquids. Viscous liquids behave differently because they resist flow more strongly.

During aspiration, a viscous liquid may take longer to enter the pipette tip. If the operator releases the plunger and immediately removes the tip from the liquid, aspiration may not have finished.

The same issue can occur during dispensing. A liquid may move down the tip more slowly and take longer to leave than a low-viscosity liquid.

This creates several practical risks:

• incomplete aspiration;
• incomplete dispensing;
• liquid remaining in the tip;
• bubbles or discontinuities in the aspirated liquid;
• inconsistent delivery between transfers.

The degree of difficulty is not determined by the label “viscous” alone. Liquid properties, temperature, transfer volume and the pipetting system can all affect behaviour. Because viscosity can change substantially with temperature, comparisons between techniques, tips or pipettes should be made under consistent temperature conditions wherever transfer performance matters.

This is why viscous-liquid pipetting should be treated as a pipette–tip–liquid workflow, rather than assuming that changing one component will solve every problem.

Start by Watching What the Liquid Is Doing

Before changing technique or consumables, observe the transfer.

After aspiration, has the liquid finished moving into the tip before you withdraw it?

Are bubbles visible in the aspirated liquid?

During dispensing, does liquid continue moving after the plunger reaches its intended position?

Is a significant amount of liquid remaining on the internal tip surface?

Are repeated transfers behaving differently from one another?

These observations help distinguish between different problems.

For example, slow movement into the tip may indicate that aspiration needs more time. Liquid coating the internal wall may indicate that surface retention is also contributing. Bubbles could result from aspiration conditions, technique or other aspects of the pipetting system.

Avoid treating every symptom as evidence that the tip itself is unsuitable.

A Practical Workflow for Pipetting Viscous Liquids

A consistent process makes it easier to identify which adjustments actually improve the transfer.

1. Use an Appropriate Pipette and Tip for the Volume

Begin with a pipette and tip suitable for the intended transfer volume.

The complete system matters. An unsuitable tip fit or an inappropriate working range can introduce variability independently of the viscosity of the liquid.

Where compatibility is important, follow the relevant manufacturer’s information rather than assuming that a tip is functionally suitable simply because it physically attaches to the pipette. Fit, sealing, volume range, filter type, sterility status, low-retention surface and chemical compatibility may all affect suitability for a specific workflow.

2. Aspirate More Deliberately

Rapid aspiration may not give a viscous liquid enough time to follow the movement of the pipette mechanism.

A slower, controlled aspiration can therefore be appropriate.

The objective is not to follow a universal “viscous liquid” speed. It is to give the liquid sufficient time to enter the tip consistently without introducing avoidable bubbles or other handling problems.

Keep the tip appropriately immersed and use a consistent technique between transfers. Changing immersion depth or pipette orientation from one transfer to the next can make it harder to determine whether an adjustment is actually improving performance.

3. Allow Time for Aspiration to Complete

Once the aspiration movement is complete, a viscous liquid may still be moving into the tip.

Allowing a short pause before withdrawing the tip can help the liquid complete its movement.

The necessary time depends on the liquid and operating conditions. Avoid treating a particular dwell time as universally correct unless it is specified for the pipette, method or application being used.

Instead, observe whether the liquid level in the tip has stabilised, where this can be seen, and apply the chosen procedure consistently.

4. Withdraw the Tip Consistently

Remove the tip from the liquid in a controlled and repeatable manner.

Abrupt or inconsistent movement can add another variable to an already difficult transfer. If liquid remains on the outside of the tip, follow an appropriate laboratory procedure for dealing with it, taking care not to touch or wick liquid from the tip orifice unless that action is part of a defined and validated method.

The important point is consistency: once a suitable procedure has been established, use the same approach for comparable transfers.

5. Dispense Slowly Enough for the Liquid to Move

Viscosity affects dispensing as well as aspiration.

A rapid plunger movement does not necessarily mean the complete intended liquid volume has immediately left the tip. Allow the liquid sufficient time to move during and after the dispense.

As with aspiration, there is no universal dispense speed or pause that is appropriate for every viscous sample.

Observe the actual liquid behaviour and use a consistent procedure.

6. Check What Remains in the Tip

After dispensing, inspect the tip where practical, recognising that visual inspection may be limited for opaque liquids, very small volumes or tips where residual liquid is difficult to see.

Residual liquid can arise for different reasons. A viscous liquid may not have completed its movement through the tip, while interaction between the liquid and the tip surface can also contribute to liquid remaining on the wall.

These mechanisms should not be treated as interchangeable.

If the principal problem is slow liquid movement, technique or the pipetting principle may require attention. If substantial liquid is coating the tip surface even after an appropriate dispense, low-retention performance may also be relevant.

7. Check Whether the Adjustment Has Improved the Transfer

For routine, non-critical transfers, consistent visual behaviour and repeatable handling may be sufficient.

Where delivered volume affects results, compare the adjusted technique under representative conditions before adopting it. This may include replicate transfers, gravimetric checking, or another laboratory-approved verification method.

Do not assume that a technique is accurate simply because the liquid appears to have left the tip. For viscous liquids, the practical question is whether the selected pipette, tip, liquid, volume and technique deliver acceptable performance for the intended method.

Forward or Reverse Pipetting?

Forward pipetting is familiar for routine liquid transfer, but reverse pipetting can be useful for some difficult liquids.

In conventional forward pipetting, the operator aspirates the intended volume and uses the pipette's dispensing sequence to deliver it.

With reverse pipetting, an excess volume is aspirated and the intended volume is dispensed while some liquid remains in the tip. This can be useful in applications where complete blow-out would make consistent handling more difficult.

For some viscous liquids, reverse pipetting can therefore be worth evaluating.

However, reverse pipetting is not automatically the correct technique for every viscous liquid.

Its suitability depends on factors including:

• the liquid;
• the pipette and tip system;
• transfer volume;
• whether the workflow can accommodate residual liquid remaining in the tip;
• the method's required delivery mode;
• the consequences of consuming excess sample or reagent.

Where sample is scarce or expensive, the additional liquid required by reverse pipetting may itself be important.

If reverse pipetting is adopted, the technique should be clearly defined and used consistently rather than allowing individual operators to interpret it differently. The procedure should also define how the residual liquid left in the tip is handled, including whether it is discarded, returned to the source container or managed by another documented method that is suitable for the sample and contamination risk.

For broader guidance on pipetting technique and reproducibility, the complete guide to pipette tips, pipetting accuracy and laboratory liquid handling provides the wider liquid-handling context.

When Should You Consider Positive-Displacement Pipetting?

Air-displacement pipettes use an air cushion between the piston and the liquid. For many laboratory liquids, this provides an effective and convenient transfer system.

Positive-displacement pipettes reduce dependence on an air cushion by bringing the piston into direct contact with the liquid, usually through a dedicated capillary or piston-tip system. They can be useful for some viscous liquids, but they still need to be selected for the required volume range, liquid compatibility, contamination requirements and manufacturer-specified operating conditions.

Some difficult liquids can make air-displacement pipetting more challenging. Where suitable performance cannot be achieved through an appropriate air-displacement technique, a positive-displacement system may be worth considering.

In a positive-displacement pipette, the piston acts directly on the liquid rather than relying on the same type of air cushion used in conventional air-displacement pipetting.

That can make positive displacement useful for certain difficult liquid properties, including high viscosity.

It should still be treated as an application decision rather than an automatic rule. The laboratory needs to consider the liquid, volume, method requirements and the performance required from the transfer.

The purpose is not to choose the most specialised pipetting system available. It is to choose a system that performs appropriately for the workflow.

Where Do Low-Retention Pipette Tips Fit?

Viscosity and surface retention are different issues.

A liquid can be difficult to aspirate and dispense because it flows slowly. It can also wet or remain on the internal surface of the tip. In some workflows, both effects may occur.

Low-retention tips are designed to reduce liquid wetting and remaining on the tip surface. They may therefore be useful where surface-related retention is materially contributing to transfer loss.

They do not make a viscous liquid less viscous or remove the need for an appropriate aspiration and dispensing technique.

This distinction matters when diagnosing a problem. If the liquid has not had sufficient time to enter or leave the tip, changing to a low-retention surface does not address the underlying flow behaviour.

If surface retention is a material part of the problem, see when laboratories should use low-retention pipette tips for the broader selection considerations.

Low-retention tips can help reduce liquid retained through interaction with the tip surface, but they do not eliminate viscosity-related flow behaviour. When selecting low-retention or filter tips, also check that the tip material, filter, sterility status and any surface treatment are appropriate for the liquid and downstream workflow.

Why Do Bubbles Occur?

Bubbles can be a warning that the aspiration process is not behaving as intended, but their presence does not identify one universal cause.

With a viscous liquid, aspiration that is too rapid for the liquid to follow can contribute to inconsistent filling of the tip. Technique, immersion conditions, pipette operation and the characteristics of the liquid can also matter.

If bubbles repeatedly appear:

1. check that the pipette and tip are appropriate for the transfer;
2. use a controlled aspiration technique;
3. allow sufficient time for the liquid to enter the tip;
4. maintain consistent tip immersion and pipette position;
5. observe whether the adjustment improves the transfer.

Avoid compensating for an unexplained problem by repeatedly changing several variables at once. That makes it difficult to establish what is actually improving performance.

Temperature Matters Too

Viscosity can change with temperature, so a liquid handled under different temperature conditions may not behave identically.

This does not mean every viscous-liquid workflow requires elaborate temperature control. It does mean that comparisons should be made under reasonably representative and consistent conditions where transfer performance is important.

If one assessment is performed with a sample under materially different conditions from routine use, the result may not accurately represent the normal workflow.

Temperature should therefore be treated as one of the conditions surrounding the transfer rather than ignored when behaviour changes unexpectedly.

How to Evaluate Whether Your Technique Is Working

Where the consequences of transfer error are significant, observation alone may not be sufficient.

A proportionate comparison can help determine whether a technique or equipment change is useful.

Define the Question

Decide what you are trying to improve.

For example:

• incomplete aspiration;
• inconsistent delivery;
• bubble formation;
• residual liquid;
• repeatability between transfers.

A clear question prevents the assessment becoming an uncontrolled comparison of several different variables.

Use Representative Conditions

Evaluate the workflow using the actual liquid where appropriate, or a suitably representative system.

A technique that works well with water cannot automatically be assumed to perform identically with a viscous sample.

Likewise, a result at one volume or under one set of conditions may not represent another workflow.

Change One Variable Where Practical

If you change the pipette, tip, aspiration speed, dispensing technique and liquid conditions simultaneously, you will have limited evidence about which change caused the result.

Where practical, compare one meaningful change at a time.

For example, a laboratory investigating aspiration behaviour might retain the same pipette, tip, liquid and volume while comparing a defined routine technique with a slower, more deliberate aspiration procedure.

Keep the Procedure Consistent

Once a technique is being compared, define it sufficiently for another operator to reproduce it.

Relevant controls can include:

• pipette and tip;
• transfer volume;
• liquid conditions;
• aspiration approach;
• pause after aspiration;
• immersion approach;
• dispensing approach;
• any blow-out or touch-off procedure.

The appropriate level of documentation should reflect the importance of the workflow.

Common Mistakes When Pipetting Viscous Liquids

Pipetting Too Quickly

A viscous liquid may not move quickly enough to follow a rapid aspiration or dispense. More deliberate operation and sufficient time for liquid movement can be more appropriate.

Removing the Tip Immediately After Aspiration

The pipette mechanism may have completed its movement before the liquid itself has finished entering the tip. A consistent pause can be useful where this occurs.

Assuming Reverse Pipetting Is Always Better

Reverse pipetting can be useful, but it has its own workflow implications and is not universally appropriate. Evaluate it for the actual application.

Assuming Low-Retention Tips Solve Viscosity

Low-retention surfaces can reduce surface-related retention. They do not change the viscosity of the liquid or eliminate the need for appropriate technique.

Changing Too Many Variables at Once

Simultaneously changing equipment, tips and technique may produce a different result, but it provides little information about why. Controlled comparisons are more useful where the workflow matters.

Using a Technique Without Standardising It

If one operator aspirates slowly and pauses while another aspirates rapidly and immediately withdraws the tip, the laboratory does not really have one pipetting procedure.

For repeat work, define the aspects of the technique that materially affect performance.

 

This sequence helps prevent a specialist tip or technique from being used as a substitute for understanding the actual handling problem.

Frequently Asked Questions

Is reverse pipetting better for viscous liquids?

Reverse pipetting can be useful for some viscous-liquid transfers, but it is not automatically better for every application. Its suitability depends on the liquid, pipette system, volume, delivery requirements and whether the workflow can accommodate the excess liquid that remains in the tip. Evaluate the technique under representative conditions before adopting it routinely.

Should viscous liquids be pipetted more slowly?

Viscous liquids often benefit from slower, more deliberate aspiration and dispensing because they take longer to move through a pipette tip. Sufficient time should also be allowed for liquid movement to complete. There is no universal speed or dwell time for every viscous liquid, so the procedure should be assessed for the actual liquid, pipette and volume.

Do low-retention tips make viscous liquids easier to pipette?

Low-retention tips can reduce liquid retained through interaction with the tip surface, but they do not remove the effects of viscosity itself. If the main problem is slow aspiration or dispensing, technique or the pipetting system may need attention. Low-retention tips are most relevant where surface-related retention is also materially contributing to the transfer problem.

When should a positive-displacement pipette be considered for viscous liquids?

A positive-displacement pipette may be worth considering when an air-displacement system does not provide suitable performance with a difficult viscous liquid. Because the piston acts directly on the liquid rather than through an air cushion, the operating principle differs from conventional air-displacement pipetting. Selection should still be based on the actual liquid, volume and workflow requirements.

Why do bubbles form when pipetting viscous liquids?

Bubbles can occur when aspiration conditions do not allow the liquid to enter the tip consistently, although they do not indicate one universal fault. With viscous liquids, overly rapid aspiration can contribute. Check the pipette-tip system, aspiration technique, immersion conditions and time allowed for the liquid to move before changing multiple variables simultaneously.

Choosing a Repeatable Approach

Accurate and consistent handling of viscous liquids is less about finding one special technique and more about matching the pipetting approach to the liquid.

Start with an appropriate pipette and tip, observe how the liquid behaves, and adjust aspiration and dispensing deliberately. Consider reverse pipetting or positive displacement where the workflow justifies them. If liquid is also being retained on the tip surface, evaluate whether low-retention tips provide a useful additional benefit.

Once a suitable approach has been established, apply it consistently. For repeat laboratory work, a defined and reproducible procedure is more useful than relying on individual operator preference.

Where surface retention is part of the problem, you can browse low-retention pipette tips and compare the available specifications against your pipette and workflow requirements.

Read More

• When Should Laboratories Use Low-Retention Pipette Tips?
• The Complete Guide to Pipette Tips, Pipetting Accuracy & Laboratory Liquid Handling

Explore More

 

Written by: Donal O’Sullivan, BSc, Co-Founder and Sales Director, LabFriend UK. Donal brings deep chemistry-led technical expertise across analytical chemistry, biochemistry, environmental monitoring, laboratory instrumentation, consumables and scientific product selection.

Reviewed by: Michael Anderson, MBA, Founder and Managing Director, LabFriend UK. Michael reviews LabFriend UK content for customer relevance, commercial accuracy, operational practicality and alignment with LabFriend UK’s laboratory supply model.

 

Browse low-retention pipette tips