Biomedical Engineering Reference
In-Depth Information
1200
1000
800
600
400
200
0
0.001
0.01
0.1
1
10
100
Sam ple Volum e (nl)
Fig. 4.2.
Predicted relation between dispensed volume and spot size: The calculated
spot sizes are determined based on the dispensed volume and the contact angle
formed between the surface and liquid as shown in the inset drawing
where
b
is the ratio between the height and radius of the sessile drop. This
value can be related to the contact angle that the droplet makes with the
surface. These parameters are pictorially described in Fig. 4.2. Also shown
in Fig. 4.2 is a graph of this relation using a contact angle of 40
◦
(such as
occurs between water and a poly-L-lysine treated glass surface). The graph
shows the relation between spot diameter and dispensed volume. Approxi-
mately an order of magnitude lowering in volume is required to drop the spot
diameter in half, and picoliter scale volumes are therefore required for spot
diameters on the order of a few tens of microns. Such volumes are in line with
reagent jet dispensing. For example, a 1200 dpi printer corresponds to spot
diameters on the order of 20
m. Alternatively, to achieve small spots, the
contact angle must be increased. Although this variable is not easily changed,
welled structures can be used to demarcate the deposition area. This requires
careful alignment between the dispenser and the target substrate, which can
be challenging when dealing with structures on the order of a few tens of
microns. This relation between volume and spot size highlights one of the
major challenges for employing any liquid dispensing technique for further
miniaturization of arrays.
µ
4.3 Thermal Jet Based Dispensing
Thermal jets, often referred to as bubble jets, eject droplets by superheating a
small volume of liquid near the dispensing orifice (Fig. 4.1a). Typically, a re-
