Correlation between clogging and sample filtration
As a classic instrument for determining liquid viscosity, the patency of the capillary tube, the core component of the U.S. viscometer directly affects the accuracy and repeatability of the measurement results. In practice, capillary clogging is one of the common faults, and according to statistics, about 90% of clogging problems can be traced back to the sample preparation stage, especially the omission in the filtration link. This article will systematically explain the causes of clogging and focus on the key control points of filtration operations.
Physical mechanism of blockage
When there are inadequately removed suspended particles or gel-like substances in the sample, these impurities are prone to adsorption or mechanical entrapment as they flow through the capillary tube (usually 0.3-0.8 mm in inner diameter). According to Poisouer's law, the capillary flow rate is proportional to the fourth power of the radius:
Q = (π·r⁴·ΔP) / (8·η· L)
where Q is the flow rate, r is the capillary radius, ΔP is the pressure difference, η is the sample viscosity, and L is the capillary length. Even local deposition of micron-sized particles can lead to a significant decrease in r, resulting in a decrease in flow, abnormal outflow time, and in severe cases, complete blockage.
Key control parameters for sample filtration
Effective filtration requires a combination of sample properties and filtration conditions. The following is a summary of common influencing factors:
| Filter membrane pore size selection | It should be less than 1/3 of the estimated minimum particle size, and ≤ 1 micron is usually recommended |
| Sample preparation | High viscosity samples need to be properly diluted or heated to reduce filtration resistance |
| Filter tightness | Avoid bypass contamination that could cause unfiltered samples to enter the filtrate |
| Filtration environmental cleanliness | Operate in a dust-free area to prevent secondary pollution of environmental particles |
| Membrane compatibility verification | Confirm that the filter membrane material does not dissolve or adsorption with the sample |
Operation process
First, the sample was pretreated with centrifugation (e.g., 3000 rpm, 10 min), and the supernatant was filtered. The filtration device needs to be rinsed with pure solvent before use. The step-by-step filtration strategy is adopted: it is pre-filtered by a larger pore size filter membrane first, and then replaced with a target pore size filter membrane fine filtration. After filtration, the clarity of the filtrate should be checked with the naked eye, and microscopic observation should be performed if necessary. Replace the filter membrane with each batch of samples to avoid cross-contamination.
Diagnosis of blockage
If the outflow time is prolonged or the repeatability deteriorates, the measurement should be stopped immediately. Reverse flushing method can be used: reverse flushing of capillaries with pure solvent at a pressure of less than 10 kPa. Instrument calibration is carried out regularly using standard viscosity solutions to establish baseline data for capillary performance. It is recommended to formulate a maintenance routine for flushing the capillary with a low-viscosity solvent after starting the machine every day.
Conclusion
Although capillary blockage is manifested as instrument failure, the root cause is mostly in the sample preparation process. By establishing strict filtration standard operating procedures, rational selection of filter materials, and strengthening operator training, the incidence of clogging can be significantly reduced and the reliability of viscosity data can be ensured. Laboratories should include the filtration process in the quality management system document and conduct regular supervision and audits.