What is dip coating?

Dip Coating Overview

Dip coating involves depositing a liquid film by precise and controlled withdrawal of a substrate from a solution. This is usually done with an instrument called a "dip coater". There are several stages in the dip coating process:

• immersion

• Residential

• quit

• drying

• curing (optional)

Figure 1 shows a simplified way of forming a liquid film when a substrate is subjected to a dip coating process. Place the substrate in the solution bath until mostly or completely submerged. A short delay occurs before the substrate is removed. During withdrawal, a thin layer of solution remains on the substrate surface. After complete extraction, the liquid in the film begins to evaporate and leaves a dry film. For some materials, a further curing step may be performed. This forces a chemical or physical change in the deposited material.

All these stages are required in the dip coating process. However, the two critical stages that determine the properties of the deposited film are the removal and drying stages.

Withdrawal and Film Formation

The exit phase of the dip coating process can be simply viewed as the interaction of several sets of forces. These forces can be classified into one of two categories: excretory forces and entrainment forces. Drainage forces work by drawing the liquid away from the substrate and away from the tank. In contrast, entrainment force is the force used to keep the fluid on the substrate. The balance between these two sets of forces determines the thickness of the wet film applied to the substrate. During the extraction phase, the wet film formation can be broken down into four regions (shown in Figure 2).

The four areas are:

1. Static meniscus - Here, the shape of the meniscus is determined by the balance of hydrostatic and capillary pressures.

2. The dynamic meniscus - this happens around the stationary point. The stagnation point is the point where the entrainment and expulsion forces are in balance.

3. Constant Thickness Region - Here, the wet film has reached a given thickness (H^0).

4. The Wet Area - This is the area where the wet film begins.

The dynamic meniscus and the flow of solution in this region determine the thickness of the wet film. Therefore, it is important to understand the physics underlying the dynamic meniscus curvature and stagnation point thickness.

Figure 3 (bottom) shows the dynamic meniscus region. The transition between the static and dynamic menisci occurs within the boundary layer (L). In this region, the forces from the viscous flow affect how the solution moves. Outside the boundary layer, the excretory force is significantly greater than the viscous force. In this region, it is the balance between hydrostatic pressure and capillary pressure that determines the meniscus.

The stagnation point occurs when the equilibrium between the entrainment force and the discharge force is equal. The balance of these forces determines the thickness of the film. The forces that determine the behavior of the coating can be defined by three different coating regimes - viscous flow, draining and capillary regimes.

粘性流失制度

第一个涂覆方式是粘性流动方式。对于高速和粘性溶液会发生这种情况。在此，涂层主要由粘性力和引力吸引。在这种情况下，液体层的厚度可以由公式1给出。

在此，夹带力由在撤出基材时作用在溶液上的粘性力组成。这由粘度（η）和基材从溶液中的撤出速度（U 0）给出。排水力是重力，由溶液的密度（ρ）和重力常数（g）给出。常数（c）与动态弯月面的曲率有关。该常数是溶液本身的性质，并且与溶液的流变性质密切相关。对于大多数牛顿液体，该常数约为0.8。

在大多数情况下，抽出速度或所用溶液的粘度不足以使这种近似有效。当减小这两个变量时，粘性力变弱。夹带力和排出力之间的平衡于是也取决于溶液的表面张力驱动的运动。正是在这些条件下，涂层才在排水范围之内。Landau-Levich方程式（参考方程式2）描述了湿膜厚度与基板的取出速度之间的关系（当考虑表面张力时）。

在考虑到非常低的提速之前，Landau-Levich方程才有效。当速度降低到低于大约0.1mm.s -1时，就会出现第三种涂层状态。这种状态称为毛细管状态。在毛细管状态下，溶液（通过粘性流）被夹带到基质上的速率低于蒸发速率。因此，干燥的动力学对于了解毛细管状态重要性无庸赘述。

干燥动力学

浸涂通常具有三个不同的干燥阶段：

1. 涂布前的干燥前

2. 该  恒速期

3. 在降速周期

简单的干燥阶段是恒定速率周期和下降速率周期。恒定速率周期发生在恒定厚度区域内（涂覆期间和涂覆之后）。在此，溶剂的蒸发发生在湿膜的表面，并在整个膜上均匀地发生。单独的例外是在发生干燥前沿的基材边缘。

随着时间的流逝，大部分溶剂将从湿膜中除去，直到形成凝胶状膜。这是下降速率周期发生的时间。在下降速率期间，残留的少量溶剂被截留在凝胶中-蒸发取决于溶剂向表面的扩散。

更复杂的干燥阶段发生在干燥前沿（如图4所示）。干燥前沿出现在湿膜和基材之间的界面上-最明显的是在润湿区域。由于更大的表面积体积，蒸发在此发生得更快，导致形成具有更高浓度的湿膜。由于表面张力的驱动作用，这导致从周围区域抽出溶液。一旦溶液在干燥前沿形成干膜，就会在溶液上施加毛细作用力。这导致溶液芯吸到干膜中-导致沉积膜变厚。

由于毛细作用，溶液被吸入干燥膜中（在毛细作用下）。对于足够快的抽出速度，干燥前沿的后退速度明显低于恒定厚度区域的形成速度。因此，干燥动力学受恒定速率周期的支配，并且最终膜厚度将取决于初始湿膜厚度。对于较慢的出纸速度（干燥前沿的后退速度快于退出速度），干燥动力学主要由干燥前沿决定。

毛细管体制

在毛细管状态下，不考虑湿膜厚度。这是因为在这些涂覆速度下永远无法真正实现恒定厚度的区域。因此，在毛细管状态下，最终厚度取决于i）抽出速率，ii）溶液的性质和iii）溶剂的蒸发速率。因此，干膜厚度由等式3给出。

在此，蒸发速率（E），涂膜的宽度（L），抽出速率（U 0）和材料比例常数（K）决定了最终的干膜厚度（h f）。K是溶质，溶液和干膜性能的组合。该常数归因于溶液（c）中溶质的总浓度，溶质的摩尔重量（M），溶质的密度（ρ）和沉积膜的孔隙率（α）。

诸如溶质的浓度，溶质的密度和材料的分子量等特性对干膜厚度具有简单而明显的影响。但是，孔隙率值明显更复杂。与原材料本身相比，孔隙率不仅会改变薄膜的密度，还会影响干燥动力学。如前所述-在干燥前部，在干膜和湿膜之间的接触点，湿膜将通过毛细作用被吸入干膜中。薄膜的孔隙率也对此有很大的影响，它决定了溶液被吸入干膜的速度，进入干膜的距离以及被吸收的材料干燥的速度。

膜厚vs退纸速度

干膜厚度作为抽出速度的函数，需要使用Landau-Levich方程和毛细状态方程。图5显示了浸涂过程的示例撤离速度与膜厚的关系图。在这两种涂覆方式之间的过渡过程中，可以实现浸涂的最小厚度。当退出速度在任一方向上都偏离最小值时，不同的涂层方式将占主导地位。

对于高速度和低速度，可以仅通过Landau-Levich方程或毛细管状态方程来给出厚度曲线。但是，对于一定范围的涂层速度，没有一个方程式可以单独给出涂层厚度的准确值。这以涂层厚度的绝对最小值发生。要计算最小厚度，您需要一个方程，该方程将Landau-Levich和毛细管状态方程统一起来。这将在下面的部分中讨论。

确定最小膜厚

为了确定最小厚度，需要将控制排水状态和毛细管状态期间的厚度的两个方程式组合在一起。第一步是修改方程式2，使湿膜厚度与干膜厚度相关。只需在方程式中包括“材料比例”常数，即可完成此操作。

将两个方程式相加得出  方程式4。在此，括号中的第一项与毛细管状态（方程式3）有关，而第二项与排水状态（方程式2）有关。在公式2中看到的常数已经转化为通用解常数（D）。

According to该方程式，可以通过相对于抽出率进行微分来确定最小的膜厚。通过将此导数设置为0（这是最小值的拐点处的梯度），您可以得到公式5。

尽管该方程式非常接近所达到的实际薄膜厚度，但在推导构成该方程式的方程式时，仍有许多因素被忽略。这些参数主要包括随时间变化的参数（由于诸如Marangoni流量，可变的蒸发速率，表面空气流量等影响）。

结论

Dip coating requires at least four distinct steps to be performed - dipping, holding, drawing out and drying. The final film thickness is determined by the interplay between entrainment force, drainage force and film drying. The membrane forms in one of three states (viscous, drained, and capillary), and transitions between them occur at varying values ​​of pumping speed and solution viscosity.

The combination of the three coating methods ultimately determines the "thickness versus shrinkage speed" behavior of the film. By summing the contributions of the drained and capillary modes, an equation is obtained that accounts for the thickness shrinkage velocity relationship over a wide range of velocities. It also allows us to determine the smallest possible thickness that can be coated for the solution.

While these equations are good approximations of actual values, many other factors that contribute to the final film formation have not been considered. These include air flow, viscosity and concentration gradients, thermal gradients and Marangoni flow.