How to make a panel blink dynamically?

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C#

ASP.NET

I have a button 'Pay' and a panel 'Panel4' which reads 'Please fill completely'. This page has
some TextBox's to be compulsorily filled in by the user. If the user does not fill all TextBoxs
then on 'Pay' button click the Panel4 is made to display. Again if the user clicks the Pay button without filling all, I want this Panel4 to blink 2-3 times.

I tried with the following code programmatically but does not blink.

protected void BtnPay_Click(object sender, EventArgs e)// pay button
    {
        if (Panel4.Visible == true)// for blink purpose
        {
            int v_blink = 0;
            int v_count=0;
            while (v_count < 220000)
            {
                v_blink = v_blink + 1;
                if (v_blink == 2100)
                {
                    Panel4.Visible = false;
                    v_blink = 0;
                }
                else
                {
                    Panel4.Visible = true;
            }
                v_count++;
            }
        }