# Source code for dclab.features.emodulus_viscosity

```#!/usr/bin/python
# -*- coding: utf-8 -*-
"""Viscosity computation for various media"""
from __future__ import division, print_function, unicode_literals

import numpy as np

#: Media for which computation of viscosity is defined
KNOWN_MEDIA = ["CellCarrier", "CellCarrierB", "water"]

[docs]def get_viscosity(medium="CellCarrier", channel_width=20.0, flow_rate=0.16,
temperature=23.0):
"""Returns the viscosity for RT-DC-specific media

Parameters
----------
medium: str
The medium to compute the viscosity for; Valid values
are defined in :const:`KNOWN_MEDIA`.
channel_width: float
The channel width in µm
flow_rate: float
Flow rate in µl/s
temperature: float or ndarray
Temperature in °C

Returns
-------
viscosity: float or ndarray
Viscosity in mPa*s

Notes
-----
- CellCarrier and CellCarrier B media are optimized for
RT-DC measurements.
- Values for the viscosity of water are computed using
equation (15) from :cite:`Kestin_1978`.
"""
# also support lower-case media and a space before the "B"
valmed = [v.lower() for v in KNOWN_MEDIA + ["CellCarrier B"]]
medium = medium.lower()
if medium not in valmed:
raise ValueError("Invalid medium: {}".format(medium))

# convert flow_rate from µl/s to m³/s
# convert channel_width from µm to m
term1 = 1.1856 * 6 * flow_rate * 1e-9 / (channel_width * 1e-6)**3 * 2 / 3

if medium == "cellcarrier":
temp_corr = (temperature / 23.2)**-0.866
term2 = 0.6771 / 0.5928 + 0.2121 / (0.5928 * 0.677)
eta = 0.179 * (term1 * term2)**(0.677 - 1) * temp_corr * 1e3
elif medium in ["cellcarrierb", "cellcarrier b"]:
temp_corr = (temperature / 23.6)**-0.866
term2 = 0.6771 / 0.5928 + 0.2121 / (0.5928 * 0.634)
eta = 0.360 * (term1 * term2)**(0.634 - 1) * temp_corr * 1e3
elif medium == "water":
# see equation (15) in Kestin et al, J. Phys. Chem. 7(3) 1978
if np.min(temperature) < 0 or np.max(temperature) > 40:
msg = "For water, the temperature must be in [0, 40] degC! " \
"Got min/max values of '{}'.".format(np.min(temperature),
np.max(temperature))
raise ValueError(msg)
eta0 = 1.002  # [mPa]
right = (20-temperature) / (temperature + 96) \
* (+ 1.2364
- 1.37e-3 * (20 - temperature)
+ 5.7e-6 * (20 - temperature)**2
)
eta = eta0 * 10**right
return eta
```